Zoo 317 Heredity, Evolution and Society


ABO blood groups   Red blood cell antigens detected by antibodies that are naturally-occurring (i.e. not produced through artifical immunization). There are three common polymorphic alleles, A, B, and O, with A and B codominant to each other and both dominant to O. Persons produce antibodies against the A and B antigens that their red cells do not have but not against O. Thus an AB person produces no antibodies, whereas an OO person produces both anti-A and anti-B antibodies. Mixing blood of persons with different ABO types results in antibodies of one person attacking the corresponding antigens of the other, resulting in agglutination of the cells. In blood transfusions, it is important that the donor and recipient have blood that is compatible. The ABO groups are the most common reason for incompatibility, and the donor and recipient are always matched for ABO types.

acentric   A chromosome or, more commonly, a chromosome fragment that lacks a centromere.

achondroplasia   A dominantly inherited form of dwarfism characterized by short limbs but normal trunk. The gene involved is the fibroblast growth factor receptor-3 gene, located at 4p16.3. More information is available in OMIM.

acquired immune deficiency syndrome (AIDS)   Impaired immunity caused by infection with the human immunodeficiency virus (HIV). The virus infects primarily the helper T-cells, eventually causing collapse of the immune response. Persons with AIDS are therefore unable to protect themselves against bacterial or viral infections.

acrocentric   [Greek akron extremity + centric.] A chromosome in which the centromere is located near one end, creating very unequal arms.

active immunity   The production of antibodies by an individual as a result of immunization, either by infection or vaccination. Active immunity creates memory cells that provide immunity throughout the life of the individual.

adaptation   The concept that organisms become better able to exploit their environment and survive in it over evolutionary time scales. This process occurs because of natural selection and involves differential survival and reproduction of better adapted gene combinations.

adenine   A purine that occurs in the nucleotides of DNA and RNA.  Commonly abbreviated A in DNA and RNA sequences.

adenosine triphosphate   Commonly called ATP, this substance stores energy in the form of chemical bonds. ATP is produced in mitochondria (also in chloroplasts in plants) and is used to drive the vast number of chemical reactions in a cell that require energy. If more ATP is produced than is required, the excess is stored in the form of fat and glycogen (or starch in plants).

agammaglobulinemia   Lack of the antibody-containing gamma-globulin protein fraction of blood. Persons who lack gamma globulin are unable to produce antibodies and therefore lack humoral immunity.

albinism   A condition in which normal pigment is absent or greatly diminished. Complete absence of pigment in humans is inherited as an autosomal recessive trait. However, there are additional forms of albinism with other modes of transmission. See OMIM for more extensive discussion and links to other forms of albinism.

alkaptonuria   A disorder in the breakdown of the amino acids phenylalanine and tyrosine. In normal metabolism, phenylalanine is converted to tyrosine, which is converted in several steps to homogentisic acid and then into other metabolic endproducts. The enzyme that catalyzes the breakdown of homogentisic acid (homogentisic acid oxidase) is coded by a single gene (AKU) on chromosome 3. Persons homozygous for defective AKU alleles cannot carry out the breakdown of homogentisic acid, which accummulates in the blood and is then excreted in the urine. On exposure to air, homogentisic acid spontaneously forms a black pigment, which is readily recognizable on wet diapers. Alkaptonuria is a relatively benign disorder, the principal problem being formation of deposits in joints, leading to a form of arthritis. More extensive information is available in OMIM.

allele   [Greek allelon of one another.] An alternate form of a gene. Some alleles are wildtype, i.e. common and normal. Others are rare and may or may not be associated with normal function. There may be hundreds of alleles for a particular gene, but usually only one or a few are common.

allergen   An antigen that produces a hyperreactive immune reponse (allergy).

Alzheimer disease   A disease of late onset involving loss of mental function and memory. It is often called senile dementia. "Early onset" cases appear before ages 60-65 years; the great majority of cases are "late onset," appearing in the seventies and eighties. Early onset cases frequently show autosomal dominant inheritance, due to the presence of mutant alleles at any of three loci. The inheritance of late onset cases is less clear and is more likely multifactorial, with genetic predisposition and environmental factors both contributing to the risk. The presence of E4 alleles at the apoliproprotein E locus is an important risk factor for late onset Alzheimer disease. Additional information is available in OMIM.

amino acid   An organic compound that has both an amino group and a carboxyl group as part of its structure. The type formula is H2–N–CH(R)–COOH, where the hydrogen (H) and the R group are attached covalently to the carbon (C) drawn to their left. The R group can be any of dozens of atomic structures, the simplest being a single H. Amino acids can have a variety of functions in metabolism, but they are especially interesting because they are the building blocks of proteins. They can be joined together in any order by peptide bonds, and there can exist a vast number of polypeptide chains that differ in their amino acid sequences.

amino group   A functional group consisting of one nitrogen and two hydrogens: –NH2.  Each H is bound to the N by a single covalent bond, satisfying the valence requirements of N for three bonds and H for one.  Because the nitrogen has unshared electrons in its outer shell, it attracts hydrogen ions, which have no electrons, and forms the structure –NH3+, which has a positive charge.  The ability to bind/neutralize hydrogen ions classifies the amino group as a base or alkaline group.

aminoacyl tRNA   The molecule that consists of an amino acid bound covalently through its carboxyl group to its specific tRNA.  These tRNA derivatives are necessary for translation of the nucleic acid code into amino acid sequences in proteins.

amniocentesis   Collecting a sample of amniotic fluid, which surrounds a fetus.  The amniotic fluid contains cells that are of fetal origin and that can be used for genetic studies of the fetus.  The optimal time for collection is about 16 weeks gestation, at which point there is enough fluid to withdraw a useful sample.  The liquid can be analyzed for various chemical constituents, and the cells can be analyzed directly by PCR or cultured for chromosome studies.  Amniocentesis is a major procedure used in prenatal diagnosis.

amniotic fluid   The liquid in which an embryo or fetus is suspended within the amniotic sac.  It consists of both liquid and cells, the latter of fetal origin.  The cells are especially useful for genetic studies.

anaphase   [Greek ana back + phase.] The phase of nuclear division in which newly formed chromosomes are pulled along the microtubules of the spindle to the opposite poles. In mitosis, former sister chromatids, now chromosomes, move to opposite poles.

anaphylaxis   An immediate hyperreactive immune response to an allergen. Symptoms may include respiratory distress and low blood pressure. Without prompt intervention, death may result.

androgen   [Greek andros man + gennan produce.] Any male hormone, of which testosterone is an example. The androgens are steroids and thus are closely related to female hormones (estrogens, progesterone) and to adrenal hormones.

androgen insensitivity   [Gene symbol AIS.] A disorder in which tissues lack functional androgen receptors and therefore cannot respond to testosterone. The condition was earlier called testicular feminization. The defect is in the gene for the androgen receptor (AR), on the X chromosome. Persons who are 46,XY but who have a nonfunctioning AR develop female external genitalia and an otherwise normal female external appearance, described in some cases as voluptuous. They lack the internal structures associated with Mullerian duct development (fallopian tubes and uterus) and the vagina is short. Some do not develop axillary and pubic hair. The intra-abdominal gonads are testes, with normal male production of testosterone. More information is available in OMIM.

androgen receptor   A cytoplasmic protein receptor that binds to androgens.  The complex acts as a transcription factor in nuclei, modifying the activities of specific target genes.  Defects in the structure of the androgen receptor interfere with this process and produce androgen insensitivity.

anencephaly  [an negative + Greek enkaphalos brain.] A condition in which the main part of the brain (cerebrum) is missing or greatly diminished.

aneuploid   Having a chromosome complement that is not a multiple of the basic haploid complement.  In humans, aneuploidy usually involves having an extra chromosome (trisomy) or deficiency of a  chromosome (monosomy).

anion   [Pronounced AN-eye-on, more or less.]  A negatively charged atom or molecule.  See ion.

antibody   A protein produced by B-cells, specifically by the plasma cells, that binds to a very specific target.  Such targets are called antigens and induce the production of the specific antibodies.  An individual can produce an enormous number of different antibodies, each specific for its target.  The diversity is possible primarily because of the rearrangements of the DNA in a limited number of coding regions, a process that is limited to antibodies and T-cell receptors. An antibody molecule is a tetrameric protein, composed of two identical large polypeptides and two identical small polypeptides. The molecular formula can thus be represented as H2L2. Combinations of different forms of the H and L polypeptides generate the enormous diversity of antibodies.

anthropoid   [Greek anthropos man + eidos form.] The "human-like" primates, including monkeys, apes, and, of course, humans. These are grouped into the suborder Anthropoidea. The remaining primates are in the suborder Prosimii.

anticipation   The phenomenon of younger age of appearance of a late onset trait in succeeding generations.  The only known cause of anticipation is trinucleotide expansion.

anticodon   The three nucleotides on transfer RNA (tRNA) that are complementary to a codon in messenger RNA (mRNA).  This complementarity binds the tRNA to the mRNA, and the attached amino acid is then transferred to the nascent polypeptide chain.  This specificity of binding of the anticodons to the mRNA codons accounts for the fidelity with which a gene is translated into protein.

antigen   Any substance that, when introduced into the body of a mammal and many other vertebrates, will induce production of antibodies.  Most antigens are proteins, but almost any substance is capable of being antigenic on rare occasions.

anti-Mullerian hormone   [Gene symbol AMH.] A hormone secreted by the Sertoli cells of the seminiferous tubules in the testes of developing male embryos and fetuses. Previously called Mullerian-inhibiting hormone/factor/substance. Its function is to suppress development of the embryonic Mullerian ducts, which form the fallopian tubes, uterus, and upper third of the vagina in females. For additional information, see OMIM.

antiparallel   Going in opposite directions, as in two arrows lying parallel but with arrow heads pointing in opposite directions.  The two polynucleotide strands of DNA have direction, and pairing occurs only if the directions are opposite.  See Watson-Crick.

antisense strand   The DNA strand that serves as the template in transcription.  Also called the template strand.

apolipoprotein E   A protein in blood that is usually bound to certain fats. There are three common alleles, one of which, E4, is associated with increased risk of Alzheimer disease. Additional information on apoE can be found in OMIM.

armadillo   [Spanish diminutive of armado, from Latin armatus.] A small nocturnal mammal of the family Dasypodidae, the icon of the Austin area because of its "live and let live" attitude. This derives from its habit of retreating into its hard armor when threatened, though this is scant protection from oncoming cars. Many persons who have lived in armadillo country for years know them only in the form of road kill. Armadillos are famous in genetics because they produce litters of four monozygotic offspring, a trick not known to occur in any other mammal.

artificial insemination   Introduction of spermatozoa artificially into the uterus rather than by sexual intercourse. This procedure is most often used when the prospective father is infertile, and another male is used as a source of semen.

asexual reproduction   Reproduction in which the products, either cells or organisms, are genetically identical to the parent. Mitosis is an example of asexual reproduction at the cell level. See sexual reproduction.

assortative mating    Selecting mates on the basis of genotype. See discussion under random mating.

atom   The smallest unit of tangible matter.  Atoms are composed of various subatomic particles, of which electrons, protons, and neutrons are the most important in understanding the properties of atoms at the level of biology.  Each atom consists of a nucleus composed of protons, each of which has one positive electrical charge, and neutrons, which are electrically neutral.  Shells of electrons, each with one negative electrical charge, surround the nucleus.  In the unreacted, neutral state, the number of electrons is equal to the number of protons.  The smallest atom is that of hydrogen, which consists of one proton in the nucleus, along with zero to two neutrons, and one electron in the shell.  It has an atomic number of one.  Increasing the number of protons changes the characteristics of the atom, changing it into a different element.

atomic mass   The mass of an atom of a particular element.  Often called atomic weight, though mass is the correct term.  One unit of atomic mass = 1.66 × 10–24 grams = 1 dalton.  This is approximately the mass of one proton or neutron.  Electrons have a much smaller mass and contribute little to the mass of an atom.

atomic nucleus   The central core of an atom.  The nucleus consists of protons and, with the exception of the common form of hydrogen, neutrons.

atomic number   The number of protons in an atomic nucleus.  Atomic numbers vary from 1 for hydrogen to 103 for the man-made element Lawrencium.  The chemical properties of an element are a function of the number of protons in the nucleus, which in turn determine the stable number of electrons in the surrounding shells.

ATP   See adenosine triphosphate.

Australopithecus   [Latin australis southern + Greek pithekos ape.] One of two hominid genera, represented entirely by fossil forms, all in east and southest Africa. The earliest species is designated A. anamensis and is represented by limited fossil material dated to ca. 4 million ybp (years before present). Another group of fossils, designated A. afarensis, are from a species that lived from ca. 4 million ybp to 2.5 million ybp. A. africanus arose perhaps as early as 3 million ybp and lasted at least to about 2 million ybp. Two other large groups, A. boisei and A. robustus, arose ca. 2.5 million ybp and lasted until nearly 1 million ybp, well past the formation of the Homo lineage. These two species are sometimes placed in a separate genus, Paranthropus. It is not clear which of these fossil species may have been ancestral to humans, except for the last two who lived too late.

autosomal   The adjective form of autosome.

autosome   A chromosome other than the chromosomes involved in sex determination. Humans have 22 pairs of autosomes + one pair of sex chromosomes.

B-cell   See B-lymphocyte.

B-lymphocyte  One of the two major types of lymphocytes, responsible primarily for humoral immunity (antibodies). B-lymphocytes were so-named because they "mature" in chickens in the bursa, a gland that is not present as such in mammals. Maturation appears instead to occur in the bone marrow. When stimulated by antigens, B-lymphocytes, which initially have little cytoplasm, become large plasma cells, which are the cells with active antibody production. Some of the B-lymphocytes also become memory cells, forming a reservoir of cells that are long-lived and that are primed to expand for antibody production in the event of later challenge with the specific antigen.

backcross   A cross between the F1 generation and the presumptive homozygous recessive parental stock. Such a cross would be Aa × aa and should produce equal numbers of Aa and aa offspring.

bacteria   [Sing. bacterium;  Greek bakterion little stick.]  One-cell organisms that lack nuclei, i.e. they are prokaryotes.  Many of the "germs" that cause disease are bacteria.  However, most are benign.  Indeed, we depend on them for many natural processes.  Several have been important experimentally in understanding the structure of genes and how they function.

bacteriophage   [bacterio- + phagein to eat.]  A virus that attacks and destroys bacteria.  Studies of bacteriophage were important in early development of the field of molecular biology.

balanced polymorphism   A polymorphic trait that is maintained as such by greater fitness of the heterozygotes as compared to either homozygote. The allele frequencies are inversely proportional to the selection coefficients of the corresponding homozygotes.

balanced translocation   A translocation in which no genetic materials are lost.  Such translocations are consistent with normal development and function except for possible interruptions of gene function at translocation breakpoints.

Barr body   See sex chromatin.

base   A molecule that has the ability to bind hydrogen ions, H+, thereby neutralizing their acidity.  Basic, in this sense, is synonymous with alkaline. While there are many basic molecules that are important in biology, those that occur in nucleic acids merit special note, as they are often referred to collectively as bases.  These are the purines adenine and guanine and the pyrimidines cytosine, thymine, and uracil.

base pair   A common designation of the pairs of nucleotides that are complementary in a DNA double helix.  As a matter of convenience, a DNA sequence is often represented by the sequence of bases on one strand.  It is understood, but frequently not stated, that each such base represents the nucleotide pair.  Thus, in writing a sequence such as -G-A-C-, the complementary sequence -C-T-G- is understood.  The context of the discussion should make clear whether a single strand of DNA is inferred or whether the double-stranded structure is the subject.

benign tumor   An abnormal growth that does not spread to or invade other tissues. As the name implies, benign tumors generally do not cause problems except for those instances in which large size interferes with function of surrounding tissues. This is especially a problem in benign brain tumors, for example, because of encasement with the skull. Some benign tumors may secrete hormones or other substances that cause physiological problems. A few benign tumors have increased likelihood of progressing to malignant tumors.

beta-amyloid   A fragment of a protein, amyloid precursor protein, that forms deposits in certain areas of the brain of persons with Alzheimer disease.

binomial nomenclature   The system of biological classification developed by Linnaeus (Carl von Linné). In the Linnaean system, the basic unit of classification is the species. Closely related species are grouped into the same genus (pl. genera). In referring to an organism by its scientific name, both the genus and species are given. For example, humans are Homo sapiens, being members of the genus Homo and the species sapiens within that genus. Related genera are grouped into families, which are grouped into orders, which are grouped into classes, which are grouped into phyla (s. phylum), which are grouped into kingdoms. Humans are members of the kingdom Animalia, phylum Chordata, Class Mammalia, Order Primates, family Hominidae, genus Homo, and species sapiens. There are rigid rules in writing these designations: The genus is capitalized; the species is not. Both are italicized, unless you are italics-challenged, in which case they must be underlined. Other levels of organization are capitalized but not italicized.

bipedal   [Latin bis twice + pes foot.] Refers to walking on two feet, therefore with upright or erect posture.

bipolar affective disorder   A psychotic disorder in which there are major mood changes between periods of mania and periods of depression. Also known as manic-depressive psychosis.

blastocyst   The early mammalian embryonic stage in which the mass of cells forms a hollow sphere. Most of the cells that form the sphere are trophoblasts. A few are embryoblasts and will form the inner cell mass.

blood group compatibility   The absence of  red blood cell antigens and the corresponding antibodies to them in mixtures of blood.  The term is also used to describe potential mismatches between mother and fetus, in which the fetus might have antigens that are not present in the mother's cells, allowing her to make antibodies against any fetal red cells that cross the placental barrier into the mother's circulation.

bond   See chemical bond.

brachiation   [Greek brachion arm + suffix -ation.] Movement by means of arms suspended from tree branches; "swinging" through trees suspended by the arms. Brachiation is especially characteristic of gibbons.

brachydactyly   [Greek brachys short + daktylos finger.]   A rare dominantly inherited condition in which the fingers and toes are short. This was the first trait in humans shown to be transmitted according to Mendelian rules (W. C. Farabee, 1903). There are several different forms of brachydactyly. Additional information is available in OMIM.

BRCA1   [Breast cancer locus 1; chromosome 17.] The first gene identified that is associated with increased risk of breast cancer. Women heterozygous for defective BRCA1 are likely to acquire the additional somatic mutations that cause progression to breast cancer. They also are at risk for ovarian cancer. Families in which defective BRCA1 is segregating typically show multiple cases of breast cancer, consistent with dominant inheritance with incomplete penetrance. For additional details, see OMIM.

BRCA2   [Breast cancer locus 2, chromosome 13.] The second locus identified that is associated with increased risk of breast cancer. In families in which variant BRCA2 is segregating, heterozygous males have increased risk of breast cancer (unlike BRCA1), but there is no increased risk of ovarian cancer (also unlike BRCA1). For additional details, see OMIM.

Brunner syndrome   An X-linked recessive trait characterized by mental retardation, impulsive and aggresive behavior, and poor judgment. The defect is in the monoamine oxidase A gene. Only one family, in Holland, has been identified with this mutation. Additional information is in OMIM.

C-terminus   The last amino acid to be added to a polypeptide chain. Starting at the N-terminus, amino acids are added one-by-one, joined with peptide bonds. The amino acids being added during chain growth always have a free carboxyl group until another amino acid is added. The C-terminal amino acid keeps its free carboxyl group, otherwise it wouldn't be the last one added, would it?

cancer   A malignant tumor. Such tumors can spread to other parts of the body and can invade other tissues.

carbon   [Symbol C; atomic number = 6; atomic mass = 12.011.]  One of the common elements, especially important in biological systems.  Carbon can form stable covalent bonds with a number of elements, generating a vast number of different kinds of molecules.  Molecules that include carbon are described as organic molecules, reflecting an earlier and false notion that they could be made only in biological systems.

carboxyl group   A functional group that consists of a carbon atom to which one oxygen is bound with a double bond and a hydroxyl group is bound with a single bond.  The remaining single bond of the carbon is attached to other atoms.  The structure is often written –COOH, with the understanding that the first O is connected to the C with a double bond and the second is connected to the C with a single bond.  An important characteristic of a carboxyl group is the propensity for the H to give up the electron that it shares with the O and become a hydrogen ion, H+.  The remaining part of the molecule would then have a negative charge.  High concentrations of hydrogen ions constitute acidity, and the carboxyl group is thus an acidic group.  Any molecule with a carboxyl group is potentially an acid. When you sip vinegar (You do, don't you?), the acid taste is due to hydrogen ions released by the carboxyl groups of acetic acid.

carcinogen   A substance that causes cancer. Use of the term is usually restricted to environmental agents, such as radiation and chemicals, although some of the products of normal metabolism may also be carcinogenic. In most cases, carcinogens are also mutagens, and it is the increase in mutation rates that account for the increase in cancer following exposure.

catalysis   [Greek katalysis dissolution.] An increase in the rate of a process in the presence of a substance (the catalyst) that itself is not consumed in the process. There are many types of catalysts. In biological systems, enzymes function as catalysts.

cation   [Pronounced CAT-eye-on, more or less.]   A negatively charged atom or molecule.  See ion.

cDNA   DNA that has been generated by reverse transcriptase acting on RNA templates, especially messenger RNA. This procedure allows mRNA to be isolated from cells or tissues and copied into DNA, thereby revealing which genes are actively transcribed in those cells or tissues. Since the template is mRNA, only the exons are represented in the cDNA. [Note: the c of cDNA stands for complementary, meaning complementary to RNA. Not the best choice for a name!]

cell   The basic unit of life. Cells can divide to produce more cells. Other structures, such as viruses and mitochondria, can do so only within a cell. Cells are surrounded by plasma membranes, which enclose cytoplasm, nuclei, and other organelles

cell cycle   A term that refers to the sequence of stages in the life of a cell, including its replication. Following cell division, a cell is said to be in the G1 phase. For most mammalian cells, this is the predominant period in the life of a cell. A cell in G1 that is not scheduled to divide again may go into a more permanent phase called G0. Cells in G1 that are ready to enter into division must first make a copy of the DNA. This period of DNA synthesis is called the S phase. When synthesis is completed, the cell is in the G2 phase for a short period. Division of the nuclear contents occurs in mitosis or meiosis, known as the M phase. At the completion of M phase, cytokinesis usually results in division of the cytoplasm, and two daughter cells are formed. Tthe daughter cells are again in G1.

cell hybrid   When two cells fuse, they form a cell hybrid. This happens rarely in nature but can be induced in the laboratory between cultured cells, even between cells from very different organisms. Thus it has been possible to make mouse/elephant hybrids and animal/plant hybrids. The two nuclei from the parental cells fuse to form a nucleus that has the full chromosome complements of both. For example, a mouse/human hybrid has nuclei with 46 human chromosomes and 40 mouse chromosomes. Cell hybrids have been very useful in assigning human genes to specific chromosomes because of the tendency in such mouse/human hybrids to lose the human chromosomes at random. If retention of a specific human gene can be associated with retention of a specific human chromosome, the locus for the gene must be on that chromosome.

cellular immunity   Specific Immunity that is based on direct interaction of receptors on the surface of T-lymphocytes with an antigen. The T-cell receptors are similar to antibodies in that they are highly diverse and can combine only with specific target antigens. As in the case of antibodies, the diversity arises largely by programmed DNA rearrangement.

cellular oncogene   See oncogene.

centimorgan   A measure of the distance between genetic markers, as measured by recombination frequency. One centimorgan (cM) = 1% recombination = 1 map unit. Named after Thomas Hunt Morgan, one of the American pioneers of genetics. See genetic map.

-centric   A combining form meaning center. As used in genetics, it often refers to centromeres. Examples: metacentric, having a centromere in the middle of the chromosome; acentric, lacking a centromere.

centromere   [Latin centrum center + Greek meros part.] The position on a chromosome at which the spindle fibers attach in cell division. The centromere is the last part of the chromosome to divide.

chemical bond   An attractive force between two atoms that holds them together.  There are several different kinds of bonds, depending on the nature of the force.  Examples are ionic bonds, covalent bonds, and hydrogen bonds.

chimera   [Greek khimaira female goat.]  An individual that results from fusion of two genetically different embryos or that has incorporated tissues from a genetically different individual.  This happens rarely in humans but has been recorded in several persons.  Several mechanisms appear to be involved, such as fusion of two zygotes, fertilization of a polar body, etc.  In the case of dizygotic twins, chimerism of the blood cells can result from joining of the blood circulation during embryonic or fetal development, with transfer of blood-forming cells from one twin to the other.  The recipient thus has two genetically different populations of blood-forming cells and may have a mixture of two blood types.  Contrast with mosaicism, in which the individual arises from a single zygote, and the differences are due to somatic mutation. See also knockout mouse.

chimpanzee   One of the great apes, a member of the genus Pan. There are two species, the common chimpanzee (Pan troglodytes) and the rare pygmy chimpanzee (Pan paniscus), both of which are native to Africa. Chimpanzees are thought to be the closest relatives of of humans, with a common ancestral line that split into the hominid and chimpanzee lines some 5 to 8 million years ago.

cholesterol   A steroid that serves as the starting point in the biosynthesis of adrenal hormones, progesterone, androgens, and estrogens, all of which are also steroids.  Cholesterol also occurs in the hydrophobic interior of plasma membranes of animals, though not of plants.  See also low-density lipoprotein, LDL receptors, and familial hypercholesterolemia.

chorionic villi   The outer layer of the embryonic and fetal membranes is the chorion.  In the early embryo, the chorion has many extensions (villi, sing. villus), some of which invaginate into the uterine endometrium to form the placenta.  Those that are not involved in this process eventually disappear.  Since the villi are derived from the same zygote as the embryo, they should be genetically identical to the embryo.  Those that are not involved in formation of the placenta can therefore be used as a genetic surrogate for the embryo in prenatal diagnosis.  They are usually collected at 8 to 10 weeks gestation, and the cells can be analyzed directly by PCR or cultured.

chrom-   [Greek chroma color.] A common prefix or suffix meaning color. Examples: chromosome, polychrome.

chromatid   One of the two replicated copies of a chromosome prior to division of the centromere. The two chromatids that originate from the same chromosomes are sister chromatids. Those that originate from two homologous chromosomes are nonsister chromatids. In mitosis, sister chromatids should be identical. Nonsister chromatids will differ in specific allele combinations, as do any pair of homologous chromosomes.

chromatin   [Greek chroma color.] The materials in nuclei that are readily stained by certain dyes. Chromatin is now known to be equivalent to chromosomes in a dispersed state. Chromosomal regions that stain very readily are designated as heterochromatin, and those that stain less are called euchromatin. Most genes appear to be within the euchromatic regions.

chromosome   [Greek chroma + soma body.] The nuclear structures in which genes are located. They consist primarily of DNA, in which the genes are encoded, and protein. A chromosomes is divided into two arms by the centromere.

chromosome library    A DNA library generated from a preparation of a specific chromosome. It is possible to sort metaphase chromosomes mechanically with high speed automated sorters. Such a preparation of chromosomes would then be digested with a restriction enzyme and the restriction fragments produced would be inserted into vectors, which in turn would be inserted into bacteria.

chromosome painting   The staining of chromosomes with fluorescent probes that are specific for individual chromosomes.  By using a variety of fluorescent dyes on the probes, one can generate chromosome spreads in which each chromosome fluoresces a different, unique color.  This technique is especially useful in detecting chromosome rearrangements.

chromosome rearrangement   The physical breakage of one or more chromosomes followed by rejoining in incorrect configurations.  Common rearrangements are deletions, duplications, inversions, and translocations.  Other, more complex rearrangements are also observed.

chromosome theory of inheritance   The proposal (1902) that chromosomes are the physical entities responsible for transmission of genes. Although the term theory suggests that other possibilities may still exist, nearly a century of research has left no plausible alternative.

class   The level of biological classification greater than orders and less than phyla. See binomial nomenclature.

clonal selection theory   The observation that the antibody response to antigenic stimulation involves large expansions of those B-lymphocytes that are already programmed to make antibodies specific for the antigen.

clone   Any group of cells or individuals that have descended from a single cell by asexual reproduction. Since no genetic recombination occurs with asexual reproduction, all members of a clone are genetically identical. See also DNA clone.

cloning vector   A plasmid, often extensively modified, that has the ability to replicate inside bacterial cells and into which DNA of interest can be inserted. Selecting such a bacterial cell and culturing it to very large populations of cells constitutes DNA cloning.

codon   A sequence of three nucleotides that code for one amino acid in a polypeptide.  Of the 64 possible codons, 61 code for amino acids and three for chain termination.

coding region   The segments of a gene that code for amino acid sequences in proteins. Introns and untranslated segments at the 5' and 3' ends of the messenger RNA are not included in the coding regions.

coding strand   The same as the sense strand.

codominant   Two alleles are described as codominant if both are expressed in a heterozygote. Example: blood group AB is produced when a person is heterozygous for the alleles for A and B.

collagen   A group of related structural proteins that are secreted by cells to form the extracellular matrix that holds cells together.  There are a number of types of collagen and collagen genes.  Mutations cause a variety of phenotypes, depending on the particular gene and on how important that gene product is in collagen formation in a particular tissue.

complementary   Descriptive of things that differ but together form a whole.  A surface of a mold and the items cast from it are complementary.  A film negative and the print made from it are complementary.  The term is used to describe the two strands of a DNA double helix.  The sequences of nucleotides on the two paired strands are different, but pairing will occur only if an adenine on one is opposite a thymine on the other, or if a guanine on one is opposite a cytosine on the other.  Thus the information on one strand, i.e. the sequence of nucleotides, determines the sequence of nucleotides on the other strand.  No additional information is present in the second strand.

complete hydatidiform mole   An abnormal pregnancy in which the implanted "embryo" consists only of placental-type structures. The embryo is completely missing. Genetic analysis indicates that the nuclear genes are entirely paternal and homozygous. This would happen if an X-bearing sperm fertilized an enucleated egg (i.e. an egg lacking a nucleus), and the haploid male pronucleus doubled to produce a diploid cell. The mitochondria are maternal, as expected. These observations are consistent with the need for both a maternal and paternal genetic contribution, presumably because of genetic imprinting.

complex inheritance   Inherited variation in phenotype that is attributed to combinations of alleles at multiple loci.

compound heterozygote   An individual who is heterozygous for two different, nonwildtype alleles.  In particular, many persons who are "homozygous" for a recessive trait are found on examination of the DNA to be heterozygous for two different alleles, neither of which is functional.

congenital adrenal hyperplasia   An inherited defect (autosomal recessive) in production of adrenal hormones. The block in biosynthesis of the adrenal hormones shifts the steroid precursors into the sex hormone pathway, increasing especially the synthesis of androgens. This causes masculinization of affected children.

congenital defect   Any defect that is present at birth, regardless of its cause.

congenital dislocation of the hip   Dislodging of the head of the femur from the hip bone, present at birth.

consanguineous   Descriptive of a mating between related persons. The offspring of such matings have increased likelihood of being homozygous for alleles because of common descent.  [This frequently mispronounced word has five syllables, with the accent on the third: con.san.GUIN.e.ous.]

constant region   The C-terminal part of the H and L polypeptides in antibody molecules. In contrast to the N-terminal regions, the antibodies from an individual have only half a dozen or so different amino acid sequences in the C-terminal region.

continuous trait   A trait whose variations are measured with a scale rather than by classification into categories. Examples: height, weight. Persons may be described as tall or short, but these generally refer to regions on the distribution curve of height rather than to distinct categories.

cosegregation   Non-independent assortment in meiosis of allelic combinations at closely linked loci.

coupled   Two alleles of interest are said to be coupled if they are on the same chromosome. See repulsion.

covalent bond   A chemical bond in which electrons of two atoms are shared, producing a stable link between the atoms.  Each element has its characteristic number of possible covalent bonds, called valence.  Among the elements of special biological interest, hydrogen can form one bond, oxygen two, nitrogen three, carbon four, and phosphorus five.  Two atoms may be connected by a single covalent bond, by a double bond, or by a triple bond, so long as the valence is not exceeded.  Atoms with fewer bonds can exist but are very unstable.

cri-du-chat syndrome   [French cri-du-chat cry of the cat.]  A syndrome that results from deletion of the distal part of the short arm of chromosome 5. The other chromosome 5 is normal. The karyotype formula is 46,XX, 5p– (for an affected female). Persons with this deletion are severely mentally retarded.  Most have an unusual cry as young infants, one that sounds to human ears like a cat meowing. Additional information and links are available on OMIM.

cross    A term, used especially in experimental genetics, referring to specific types of matings.

crossing over   Exchange of chromosomal segments between nonsister chromatids of homologous chromosomes that occurs during prophase I of meiosis. This generates new chromosomal combinations of alleles.

cystic fibrosis   An autosomal recessive disorder in humans that results from impaired ability to regulate chloride transport across plasma membranes. This interferes with secretion of various glands, with loss of salt in the sweat and inability to clear the lungs of mucous. The last, in particular, leads to respiratory infections. Children with cystic fibrosis usually do not survive beyond the second decade, and few survive the fourth decade. In spite of this, the disease is rather common among populations of European descent, with a frequency of ca. 1/2500 live births. Thus, ca. one person in 25 is heterozygous for a CF allele. It has been suggested that heterozygotes have a survival advantage that offsets the lethality of CF homozygotes. There is evidence that the advantage may be survival of typhoid fever. Additional information on CF is available in OMIM, as are links to other resources.

cyto-, -cyte   [Greek kytos hollow vessel] Prefix and suffix forms for cell.

cytokinesis   [cyto- + Greek kinesis motion.] The process of division of the cytoplasm in the formation of two new cells.

cytoplasm   The part of a cell enclosed by the plasma membrane but excluding the nucleus.

cytosine   A pyrimidine that occurs in the nucleotides of DNA and RNA.  Commonly abbreviated C in DNA and RNA sequences.

cytoskeleton   A network of fiber-like structures in the cytoplasm that provide form for the cell and that may have other functions also.

cytotoxic T-cell   See T-lymphocyte.

Darwin   Charles Darwin (1809-1882), along with Alfred Russel Wallace, proposed in 1858 that natural selection is the driving force of evolution. His 1859 publication, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, is regarded by many as the most influential book of the Nineteenth Century, though actually read by few. Its premises permeate all biological thought today and have been used by diverse social and political groups to support their views, even those that are diametrically opposed, such as Marxism and unbridled capitalism.

degenerate code   Any code in which more than one symbol or combination equates to the same item.  The genetic code is degenerate because the same amino acid may be coded by more than one codon.  This is not surprising, because only 20 amino acids are coded by 61 codons.

deletion   Loss of a segment of a chromosome.  Common mechanisms are (1) two breaks in a chromosome, with loss of the middle segment prior to joining and repair of the two end segments; (2) unequal crossing over, which produces a deficiency (deletion) in one crossover product and a duplication in the other. The size of the deletion can be a single nucleotide or a large number of nucleotides that includes many genes.

denature   To disrupt the normal (native) structure of a protein or DNA. In the case of proteins, the tertiary and secondary structures are destroyed, typically by heat or acid, leaving only the primary structure held together by covalent bonds. In the case of DNA, the hydrogen bonds that hold the double helix together are broken, leaving single strands of DNA. The conditions used to denature proteins and DNA are not sufficient to break covalent bonds but can break other types of bonds. Cooking an egg is an example of converting native proteins, which are soluble, into denatured proteins, which are insoluble.

dental arch   The form of the tooth row. During hominid evolution, the dental arch has changed from U-shaped to a more paraboloid shape.

deoxyribonucleic acid (DNA)   The molecule that serves as the primary repository of genetic information. DNA occurs almost exclusively in the nuclei of eukaryotes, with very small amounts in mitochondria and, in plant cells, in chloroplasts. DNA is a polymer of deoxyribonucleotides. The bases that occur in DNA are adenine, guanine, cytosine, and thymine. In cells, DNA most often occurs as a double-strand structure, as proposed in the Watson-Crick model.

deoxyribose   A five-carbon sugar that occurs in the nucleotides of DNA.  As the name suggests, deoxyribose has one fewer oxygen atoms in its structure than does ribose.

development   "Development is a continuous process that begins when an ovum is fertilized by a sperm and ends at death. It is a process of change and growth which transforms the zygote, a single cell, into a multicellular adult human being." [K. L. Moore, The Developing Human, Saunders, 1973, p 1.] Included in this definition is differentiation, the generation of different kinds of tissues from a zygote, and organization of the different tissues into functioning organs.

diabetes mellitus   [Latin diabetes siphon + mellitus honey.] A condition in which lack of regulation of blood sugar (glucose) leads to a variety of metabolic problems. The term diabetes refers to the high volume of urine produced, and mellitus refers to the sweetness caused by glucose. The basic problem is lack of production of insulin by the pancreas or lack of effectiveness of insulin produced. Diabetes mellitus can be classified into type I and type II. Type I is typically early onset, in the early teens, but may occur at any age. The cells in the pancreas that produce insulin are destroyed by antibodies in an autoimmune reaction, thought to be induced by infection of those cells by a virus. Affected persons are dependent on external sources of insulin for survival, and this form is sometimes called insulin-dependent diabetes. Type II typically appears in late middle age but can appear much earlier. Insulin is still produced but not in necessary quantities nor does it regulate glucose effectively. The disease can often be managed purely by diet, but frequently insulin supplements are required. Type II is also known as insulin-independent diabetes, though that is a misnomer for some patients. The risk of acquiring either type I or type II is in part hereditary. In the case of type I, certain haplotypes of the MHC complex predispose to developing the disorder. In the case of type II, specific genes have not been identified, and the risk is probably multifactorial. There is strong clustering in families, however, including successive generations, which suggests dominant effects. There is also marked differences in frequency of type II in different populations, Native Americans being especially at risk. The high frequency of type II in many populations has led to the suggestion that type II may be the result of "thrifty" gene(s), that is, it may have had selective advantage under primitive hunter-gatherer societies in which life tended to be feast or famine. Additional details are available in OMIM.

Diego blood groups   A polymorphic antigenic system on the surface of red blood cells due to a locus on chromosome 17. Two codominantly inherited antigens are recognized: Di(a) and Di(b). Di(a) was first found in Venezuela in a person with American Indian ancestry. Subsequently, it was found that Di(a) occurs primarily in Asians and American Indians and is found rarely in other populations, who typically have only Di(b). Additional information can be found in OMIM.

differentiation   The generation of different tissues from one or a few cells. Differentiation involves turning some genes on and others off in a programmed manner, often strongly influenced by signals from other cells. A zygote is described as "totipotent," meaning that it has the potential to differentiate into any cell type. This is also true of many cells of a very early embryo. As differentiation proceeds, however, cells of mammals typically lose totipotency. At least, we do not know how to reverse differentiation. The great significance of cloning, a la Dolly, is that differentiation was reversed. However, we still do not understand differentiation at the molecular level, and it remains to be seen whether cloning can be carried out using nuclei from a variety of tissues and whether the conditions for reversing differentiation can be defined at the molecular level and extended.

dihybrid cross   A cross, particularly in experimental genetics, between two parental stocks that differ at two genetic loci of interest. Dihybrid crosses are useful in determining whether the two loci segregate independently.

diploid   Having two sets of chromosomes. In humans, diploid cells have 46 chromosomes (two sets of 22 autosomes plus a pair of sex chromosomes, either two X chromosomes (females) or one X and one Y (males). All somatic cells are diploid. Germ cells are diploid prior to the first division of meiosis but are haploid following the first meiotic division.

direct gene count    Counting the alleles in various genotypic combinations in order to establish the overall allele frequencies. In order to do a direct gene count, each genotype must be recognizable. This occurs only if all alleles are codominant.

discontinuous trait   A trait whose variations fall into distinct categories rather than positions on a continuous scale. Examples: male/female, presence or absence of an extra digit.

disjunction   The separation of homologous chromosomes during mitosis or meiosis. (In meiosis II, the separation involves sister chromosomes.) Disjunction assures that each daughter cell receives an equivalent complement of chromosomes. An error in chromosome separation (yes, it happens!) is called nondisjunction.

dispermy   The fertilization of an ovum by two sperm.  The resulting zygote is triploid.

distal   Refers to some event that is farther away from the centromere than some specified marker or landmark.

dizygotic   The term applied to twins who are derived from two separate zygotes. They are genetically dissimilar and are also called fraternal twins or DZ twins. They arise when two or more ova are released at the same time and are fertilized. They have the same genetic relationship to each other as any pair of siblings. DZ twins cluster in families to some extent. Therefore the tendency for DZ twinning appears to be inherited in part. The use of "fertility" drugs increases the release of multiple ova and therefore of dizygotic twins and other multiple births.

DNA   See deoxyribonucleic acid.

DNA clone   A bacterial cell, usually E. coli, into which foreign DNA has been inserted, typically by means of a plasmid (cloning vector). Very large quantities of the inserted DNA can be prepared in this way.

DNA fingerprint   A term used originally to describe a genetic profile based on certain minisatellite genetic markers. If a sufficient number of markers is used, a particular combination of markers would be as characteristic of an individual as a fingerprint (except that monozygotic twins would have identical genetic profiles, though not quite identical fingerprints).

DNA hybrid   A double-strand DNA segment in which the two strands are from different sources. The strands must, of course, be complementary.

DNA library   A culture of genetically engineered bacteria into which have been incorporated vectors that contain restriction fragments from a specified source of DNA. The cells would need to be cloned, i.e. spread on solid medium at high dilution so that colonies arise from individual cells. Colonies are then tested individually to locate those with particular DNA sequences inserted. The term library is appropriate in the sense that the culture has a huge variety, sometimes hundreds of thousands of bacteria, each distinguished by its foreign DNA. But it is like a library in which the books are shelved at random. Finding the right book, or bacterial colony, depends on identifying the colonies that bind a specific DNA probe.

DNA ligase   An enzyme that catalyzes formation or rejoining of a covalent bond between adjacent nucleotides when one of a pair of complementary strands has a gap.

DNA polymerase  An enzyme that catalyzes the assembly of DNA polynucleotides from the four deoxyribonucleotides. In DNA replication, the full name of the enzyme is DNA-dependent DNA polymerase, reflecting the fact that the template is DNA. RNA can also serve as a template if RNA-dependent DNA polymerase (commonly called reverse transcriptase) is present. DNA polymerase catalyzes the addition of nucleotides only to the 3' end of a polynucleotide strand that is complementary to and already bound to the DNA template.

DNA primer   A short segment of DNA that is complementary to one end of a DNA strand to be copied and that provides a 3' end to initiate assembly of the new strand by DNA polymerase. The primer would thus form the 5' end of the new strand.

DNA probe   A short segment of single-strand DNA that is labeled and that can be used to detect the presence of DNA that contains a sequence that is complementary to the probe. The label often is 32P (a radioactive isotope of phosphorus) but fluorescent labels also are now available. The size of the probe may be as few as 10-20 nucleotides or may be several hundred. DNA probes are used to test genetically engineered bacterial colonies to see which have incorporated a specific DNA fragment, to show the presence of a specific fragment in a mixture of fragments (Southern blot), or to show the presence and location of a specific gene on a chromosome (FISH).

DNA profile   A list of the alleles present at genetic marker loci in the DNA of an individual. See DNA fingerprint.

DNA sequence   The order in which nucleotide pairs occur in a segment of DNA. There are several methods for establishing a DNA sequence. The most prevalent methods are based on making copies of a DNA strand using DNA polymerase as the catalyst. In the presence of small amounts of a nucleotide analog, usually one that lacks a 3' hydroxyl, the analog will be incorporated but extension of the strand stops. This indicates which nucleotide is present at that position.

dominant   An allele that is expressed phenotypically when in combination with a different allele.

double helix   A helix formed by two strands intertwined to form a spiral.  The common form of DNA is a double helix.  See Watson-Crick.

Down syndrome   A syndrome due to trisomy for chromosome 21; also called trisomy-21 syndrome.  Affected persons are mentally retarded and have various other features, including somewhat short stature, epicanthal folds, characteristic fingerprints and flexion creases in the palms, and large tongue.  Most die in utero, but those who are liveborn have good life expectancy.  They have high risk of leukemia, one of the frequent causes of death, and they are subject to respiratory infections, another frequent cause of death.  Some 5% of cases are due to unbalanced chromosome translocations rather than simple trisomy.  The imbalance generates three copies of the long arm of chromosome 21, leading to the same gene dosage imbalance as trisomy 21.

ductus deferens   The duct that leads from the epididymis and through which spermatozoa pass, joining with secretions of the seminal vesicles to form semen.

Duffy blood groups   A polymorphic antigenic system on the surface of red blood cells, coded by a locus on chromosome 1. Most populations have two common alleles, FY*A and FY*B, that are expressed codominantly. African groups have a very high frequency, often approaching 100%, of a third allele, FY, that is associated with lack of antigen. The Duffy blood groups are the receptors on red cells that allow infection by the malarial parasite Plasmodium vivax. Africans who are homozygous for the FY allele are resistant to this form of malaria. Additional information is available in OMIM.

duplication   A repeated chromosomal segment.  The most common way of producing a duplication is by unequal crossing over.  This produces a chromosomal status sometimes called partial trisomy if the duplicated segment is large.  The phenotypic effects of duplications depend on the specific imbalance of genes created by the duplication.

electron   A subatomic particle that has a negative electrical charge.  The unit negative charge on one electron is equal to the unit positive charge of one proton.  Electrons occur in shells surrounding the atomic nucleus.  They form the various types of chemical bonds that join atoms.

electrophoresis   The separation of charged molecules (ions) in an electric field. Under the alkaline conditions ordinarily used, DNA and most proteins have a negative electrical charge (anions). They move toward the positively-charged electrical pole and away from the negative pole. Molecules with different charge densities separate from each other because of their different rates of movement. Gel electrophoresis is a variation that is especially useful in separating DNA or protein molecules. The gel forms a matrix that acts somewhat like a sieve, separating molecules on the basis of size. In the case of DNA, the charge density is uniform, and size alone separates different polynucleotides, even those that differ by a single nucleotide. The mobility of proteins in gel electrophoresis is dependent both on charge and size.

element   A basic unit of matter, consisting of atoms with the same atomic number. Over 100 elements are known, some of the largest being manmade and very unstable.  Only a portion are important in biological systems, and most biological molecules are built only from some half dozen different elements.  Among the most common elements in biological systems are carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur.

elliptocytosis   A rare autosomal dominant trait in which red blood cells have an elliptical shape rather than the usual biconcave disk shape.  The defect is due to mutations in any of several genes that code for proteins that normally interact to form a skeletal structure on the interior of the red cell membrane.   Descriptions of the various genes involved can be found in OMIM.

embryo   The initial stage of development. In mammals, the term is applied to the inner cell mass through the point at which all major structures are present (the end of the seventh week in humans). After that point, the developing organism is called a fetus.

endocytosis   The transport of materials into a cell by their binding to receptors, which then form a concave "bubble" into the cell.  Since the transported materials do not have to pass through a plasma membrane, they can be of quite large size.  The process is facilitated by the presence of specific receptors in the plasma membrane that bind the substances to be transported.

endoplasmic reticulum   Membranous structures that occur in the cytoplasm of eukaryotes. Many are heavily loaded with ribosomes (rough endoplasmic reticulum) and are the sites of protein synthesis.

endoreduplication   Replication of chromosomes without subsequent division of the cell.  The result, in the case of a diploid cell, is a tetraploid cell.

enzyme   A biological catalyst. The great majority of catalysts are proteins and are thus the product of genes. Some RNA molecules are also known to have catalytic activity (ribozymes). Each enzyme is highly specific in the kind of reaction that is catalyzed and may be very specific in the substrate on which it acts. The ending -ase is indicative of an enzyme. For example, a polymerase is an enzyme that catalyzes the formation of polymers from the subunits. See DNA polymerase. Some of the earliest enzymes identified were given names prior to the "-ase" rule, and the old names are still used, e.g. trypsin, pepsin.

epididymis   The tubular structure into which spermatozoa pass from the seminiferous tubules for maturation and storage. They pass from the epididymis into the ductus deferens (vas deferens).

epigenesis   The idea that embryonic development occurs by the production of complex structures from simple structures. Epigenesis is the opposite of preformation. The current view that the fertilized egg carries instructions for the development of complex organs and tissues is an example of epigenesis.

Escherichia coli   The common bacterium of the colon.  E. coli is also the most widely used bacterium in laboratory studies of prokaryotes because of ease of cultivation and the fact that it is not associated with disease.  (Recent news reports have given a bad name to E. coli because of pathogenicity of some exceedingly rare strains.  But everyone reading those reports already has a gut full of the common, benign strains.)  Because E. coli has been used in so many experiments, more is known about it than any other bacterium.  Therefore, it was selected as one of the target organisms for complete DNA sequencing.  The sequence was completed in 1998, making it one of the few organisms for which that is true.

estrogen   [Latin oestrus, Greek oistros gadfly, passion, frenzy] Any of several related steroids that promote development of female genitalia and other sexual characteristics and that are increased in the menstrual cycle preceding ovulation. Estrogens are produced primarily in the ovaries but are also produced in both men and women by the adrenal glands.

eu-   [Greek eu good.] Prefix meaning well, good. Examples: eukaryote, having a true nucleus; euploid, having a balanced chromosome complement, i.e. n, 2n, 3n, etc.

eugenics   The improvement of the population by "good genetic practices." The eugenics movement started in the late 19th Century, primarily in the U.K., and spread to Germany and the U.S. The basic premise was to encourage persons with desirable genetic traits to have lots of children and to encourage those with undesirable genetic traits not to reproduce. Far too little was known about the inheritance of human traits to allow any such program to be successful, quite apart from the moral and ethical issues raised.

eukaryote   [Also spelled eucaryote.] Cells or organisms whose cells have nuclei. All plants and animals are eukaryotes. Organisms such as bacteria whose cells lack nuclei are prokaryotes.

euploid   Having a "balanced" complement of chromosomes.  In practice, this means having a multiple of the basic haploid set of chromosomes, e.g. n, 2n, 3n, 4n, etc.  See aneuploid.

evolution   The change over time of the genetic constitution of a population. The forces that lead to evolutionary change are natural selection and random genetic drift.

exon   The segments of RNA in the primary transcript that remain after splicing to form messenger RNA.

expression library   A DNA library based on vectors that have the necessary promoter sequences to cause transcription and translation in the bacterial host. For example, the gene for human growth hormone in an expression vector would produce the protein product, not just additional copies of the DNA.

expressivity   The phenotypic variation that is associated with a specific genotype.

F1   The first filial generation. The designation for the offspring of a cross between two parental stocks.

F2   The second filial generation, ordinarily produced by intercrossing the F1 generation among themselves.

fallopian tube   The pathway that leads from the ovary to the uterus. Ova released by rupture of an ovarian follicle enter the fallopian tube, where they may become fertilized if spermatozoa are present. The developing embryo then migrates over a period of several days into the uterus, where implantation in the endometrium occurs.

familial hypercholesterolemia (FHC)   A dominantly inherited condition that causes high levels of plasma LDL (low-density lipoprotein).  The problem is caused by defective LDL receptors.  This interferes with cell uptake of LDL, which in turn leads to greater synthesis of cholesterol.  Persons who are heterozygous for an allele that causes defective LDL receptors have greatly elevated blood cholesterol, with high risk of coronary disease.  Most have major heart attacks in middle age, some as young as the thirties.  The frequency of heterozygotes in the general population is ca. 1/500 persons.  Therefore, this condition is not the major cause of high cholesterol and heart disease.  Homozygotes are rare, ca. 1/250,000, and have much higher levels of blood cholesterol than do heterozygotes, even as very young children.  They are at high risk of heart disease during childhood, and few survive.  Homozygosity for FHC is one cause of the need for heart/liver transplants, the heart because of the damage that has occurred and the liver because that is the primary organ in which cholesterol synthesis occurs.  Liver transplants in this case are a form of gene therapy, even though it be low tech by genetic engineering standards.

family    The level of biological classification greater than genera and less than order. See binomial nomenclature.

fetal alcohol syndrome  A condition in newborns due to large consumption of alcohol by the mother during pregnancy. Such children have a typical facial appearance, including a flat nose and wide-set eyes, and they are mentally retarded.

fetus   The developing organism, from the embryonic period to birth. In humans, the fetal period begins at the eighth week of gestation.

fibrillin   [Gene symbol FBN1]  A protein that is an important component of the extracellular matrix in connective tissue. Defects in fibrillin are responsible for Marfan syndrome. The FBN1 locus is on chromosome 15.

fibroblast   A rather generalized cell type found widely distributed in various tissues of mammals. Fibroblasts grow well in the laboratory, which, with their wide distribution, makes them very useful in chromosome studies.

fitness   The ability to leave one's genes to future generations. See selection coefficient.

5'-flanking region   The region of DNA adjacent to the 5' end of the coding region of a gene. This region, outside the coding region, regulates the rate of transcription.

fluorescence in situ hybridization (FISH)   A technique for revealing specific DNA sequences, applied to chromosome spreads and tissue sections.  The technique uses a probe that consists of a DNA segment to which is attached fluorescent dyes.  The DNA in the chromosomes or tissue sections is denatured, usually by heat, to separate the two DNA strands.  The probe is added, and conditions are adjusted, usually by lowering the temperature, to allow reannealing of complementary DNA.  Because of excess of probe, any DNA that is complementary to the probe will reform double strands with the probe rather than the original DNA partner.  Binding of the probe is detected by the fluorescence of the attached dye.  This method is very sensitive and can be used with a chromosome spread to detect single genes.

folic acid  One of the B vitamins, essential in the diet. Folic acid deficiency greatly increases the risk of neural tube defects in developing embryos.

foramen magnum   The opening at the base of the skull through which the spinal cord enters the cranial cavity.

fossil   A remnant or impression in rock of an ancient organism. Most fossils are the remaining hard parts, e.g. bones, of organisms that have been penetrated by minerals with subsequent hardening to rock-like structures, preserving the form of the original.

founder effect   A shift in gene pool that occurs when a small number of individuals form a new population. The shift occurs purely by chance because the founding members of the new population do not happen to match exactly the allele frequencies of the parental population.

fragile site   A marked constriction in a chromosome that occurs under certain conditions of cell culture.  Also called a secondary constriction, the primary constriction being the centromeric region.  Rather than being fragile, the sites are regions of diminished chromosome coiling, giving the appearance of fragility.  Some fragile sites are very common; others are rare.  Only two are associated with phenotypic effects: fragile-X A (FRAXA) and fragile-X E (FRAXE), both near the end of the long arm of the X chromosome.  FRAXA is associated with marked mental retardation in hemizygous males and marginal retardation in heterozygous females.  FRAXA is due to trinucleotide expansion in the untranslated 5' region of the FMR1 (familial mental retardation 1) gene on the X chromosome.  Individuals with the "premutation" form of the gene are phenotypically normal.  If they are females, they have a high risk of transmitting the "full mutation" form of the gene.

fragile-X syndrome   A condition that results from the full mutation of the fragile-X locus (FRAXA) near the end of the long arm of the X chromosome.  In males, the syndrome includes marked mental retardation, a typical facial appearance, and macrorchidism (very large testes).  In heterozygous females, there is mile mental retardation.  The fragile-X syndrome commonly occurs as a result of trinucleotide expansion in the 5' untranslated region of the FMR1 locus.  Rare cases have been reported due to conventional mutations in the coding region of the gene.  Apparently the trinucleotide expansion leads to greatly reduced translation of an otherwise normal FMR1 messenger RNA.

frameshift mutation   A mutation that alters the translation reading frame.  This happens when nucleotides (except for multiples of three) are inserted or deleted within the coding region of a gene.  From that point on in the translation, the new reading frame generates amino acid sequences different from the original.

functional group   Two or more covalently-linked atoms that often occur together and that confer a particular chemical property to the molecule of which they are a part.  Examples of functional groups include carboxyl groups, hydroxyl groups, and amino groups.

fusion gene   A gene that is a composite of two genes, generated usually by translocation.  One mechanism for inappropriate expression of a gene is chromosomal translocation that places the coding region of the gene under control of the promoter of another gene. In some instances, the translocation breakpoint is within the coding regions of the two genes, and the resulting protein may have an N-terminal amino sequence that corresponds to the promoter but a C-terminal sequence that is from another gene. If the bulk of the fusion protein is from an oncogene and is functional in the cell, the consequences may be similar to overexpression of the original oncogene.

galactose   A sugar, similar to glucose, that is found in very low quantities in certain tissues of the body. It combines with glucose to form lactose, the sugar in milk. An inherited defect in metabolism of galactose causes galactosemia.

galactosemia   An autosomal recessive disorder caused by lack of the enzyme galactose 1-phosphate uridyl transferase. As a result, galactose 1-phosphate accummulates and interferes with other metabolic processes. Galactose occurs in the diet in the form of lactose, which is the sugar in milk. Infants placed on milk therefore are most subject to the toxic effects. These include severe mental retardation, liver damage, cataracts, and death. Removal of milk from the diet greatly reduces the problem. Galactosemia is very rare and should not be confused with the common condition of lactose intolerance. More information is available in OMIM.

gamete   [Greek gamete wife; gametes husband.] A haploid germ cell, either maternal (ovum) or paternal (sperm).

gene   A unit of heredity. Originally, gene was used to describe a unit of phenotypic variation. It is also now used to describe a unit of function, i.e., a segment of DNA from which a single RNA molecule is transcribed. Ultimately, the two definitions merge as more knowledge is gained of the molecular basis of hereditary variations.

gene amplification   The condition in which a gene is copied many times, often in tandem, though not always. Even though a gene may be switched "off," there is often a very low rate of transcription. With hundreds or thousands of copies of the gene, the overall result may be substantial transcription. If the amplified gene is an oncogene, this can cause loss of regulation of cell growth. Gene amplification is found in many cancers, especially in the more advanced stages.

gene frequency    The frequency of specific alleles in a population, without regard to how they are combined in individuals.

gene pool    The alleles in a population, viewed collectively rather than combined in individuals.

genetic code   The sequence of DNA nucleotides that are translated into amino acid sequences in proteins.  The code is a triplet code, i.e. a sequence of three nucleotides codes for one amino acid.  This unit of three nucleotides is called a codon.  Of the 64 possible sequences of three nucleotides, 61 code for amino acids.  The remaining three are stop codons, that is, when a stop codon is encountered during translation, synthesis of that polypeptide chain is terminated.  The genetic code is universal, in the sense that the same code is used by all forms of life, from viruses and prokaryotes to plants and animals.

genetic distance   The distance between two genetic markers as measured by recombination frequency. See genetic map.

genetic drift   The shift in the gene pool of a population that is the result of purely random factors. The most important factor is small size of a population, which can lead to chance alterations of allele frequencies in succeeding generations. Genetic drift is to be contrasted with natural selection, in which variations in fitness among genotypes alter the gene pool.

genetic engineering   A general term that refers to the assembly of DNA from different sources into single pieces of DNA, which then are cloned in bacteria or other vectors or inserted into the genome of an organism by some means.

genetic equilibrium    See Hardy-Weinberg equilibrium.

genetic heterogeneity   A general term that reflects the fact that a particular phenotype may be due to different loci or to different allelic combinations at a particular locus. For example, the dominant trait elliptocytosis is due to a locus on chromosome 1p in some families and to a locus on chromosome 1q in others.

genetic map   A representation of the location of genes or genetic markers with respect to each other, based on recombination frequencies. A genetic map is linear, and distances between loci are measured in recombination percentage = map units = centimorgans. Contrast with physical map.

genetic marker   Any gene or other DNA variation that is useful in tracking some genetic event. For most purposes, the marker must be polymorphic. In the case of a gene used as a genetic marker, it is the variation that is of interest, not the gene function. Example: A blood group O man cannot be the father of an AB child. In this example, the function of the blood groups is irrelevant. It is the rules of their transmission that provides an answer to the question posed. With the development of new DNA technology, the majority of genetic markers are variations in DNA at sites that may or may not be part of a gene. However, their transmission to offspring follows Mendelian rules for codominant inheritance. They can be mapped just as genes are mapped.

genetics   The study of heredity.

genome   A haploid complement of genes, used particularlyi in the abstract. Common usage also refers to the diploid complement of diploid organisms. ("He has a gene for stupidity in his genome.")

genome screen   A search for association or cosegregation of a trait with genetic markers that are closely spaced throughout the entire genome.

genomic library   A DNA library generated from DNA fragments produced from the entire DNA (genome) of an organism. Nongenomic libraries would start with some fraction of the DNA, such as a particular chromosome or other large piece of DNA.

genotype   The genetic constitution of an organism, irrespective of the extent to which the genes are expressed in the phenotype.

genotypic (genetic) ratio   The ratio of genotypes among the offspring of a genetic cross, irrespective of whether the genotypes are distinguishable phenotypically.

genus   [pl. genera] The level of biological classification greater than species and less than family. See binomial nomenclature.

geographic isolation   Reproductive isolation that is due to geographic separation. Oceans, large rivers, deserts, mountains, etc., can be effective barriers to genetic exchange between populations.

germ cell   Ova and sperm, or a cell that has the potential to undergo meiosis and become an ovum or sperm. Germ cells are set aside early in mammalian embryogenesis. An event, such as mutation, must occur in a germ cell in order to be potentially transmissible to offspring. All cells other than germ cells are called somatic cells.

germinal mutation   A mutation that occurs in germ cells and that therefore can be transmitted to offspring.

gibbon   A small anthropoid ape, found in Asia. The gibbons are adapted for arboreal life, with long, thin limbs, the arms used for moving through the trees by brachiation. The gibbons are the most distant from humans of the anthropoids.

glucose   A common sugar in all forms of life. Glucose is the major metabolic link between carbohydrates and fats. It is metabolized in cells, generating energy in the form of ATP. Glucose molecules are joined together to form starch in plants and glycogen in the livers of animals, both of which serve for energy storage. Glucose also polymerizes to form the structural (woody) component cellulose in plants.

glucose 6-phosphate dehydrogenase (G6PD) deficiency   Inherited deficiencies of the X-linked gene that codes for G6PD. These deficiencies are common in Africans and in Mediterranean (Italian, Greek) populations, where the high frequencies are thought to be due to resistance of G6PD-deficient persons to falciparum malaria. Deficiency appears to be due primarily to reduced stability of the enzyme, which must function without replacement for the average 120 day life of a red blood cell. G6PD occurs in all cells, though replacement occurs to some extent in all but red cells. Deficiency in Mediterranean populations is due to an allele G6PD*MED, which produces severe deficiency, including tissues in addition to red cells. The condition known as favism results when such persons consume faba beans. There is mass destruction of red cells and damage to other tissues, often resulting in death. The African populations primarily have G6PD*A–, which has a less severe potential for unpleasantness. The gene was first detected when African-American troops returning from the Pacific during World War II were given the synthetic antimalarial primaquine. Destruction of red cells occurred to some extent, followed by return to normal. Either allele creates a potential for sensitivity to a variety of drugs and other chemical agents. In African populations, there is also another polymorphic allele, G6PD*A+, which is a normal variant whose product is separated by electrophoresis from the product of the common G6PD*B allele. Since G6PD is on the X chromosome, only males, who are hemizygous for the X, are affected with the deficiencies. Heterozygous females sometimes show degrees of deficiency consistent with chance bias in inactivation of the normal X chromosome. The polymorphism of the G6PD locus has made it a useful genetic marker, first to support experimentally the hypothesis of X-chromosome inactivation in females, and second to prove that tumors are monoclonal. Further information on G6PD is available in OMIM.

glycoprotein   A protein to which sugars have been attached covalently. The attachments are to specific amino acids at specific sites in the polypeptide chain, and attachment occurs after translation. Sugars are very hydrophilic. The sites of attachment thus become very hydrophilic.

Golgi complex   A membranous organelle that is involved in processing of newly synthesized proteins, especially those that are secreted from the cell.

gonad   A testis or ovary.

gonial cell   Cells in the germ line that replicate by mitosis. In males, such cells are spermatogonia; in females they are oögonia. Once a gonial cell enters meiosis, it is designated a spermatocyte or oöcyte.

gorilla   One of the great apes, found in Africa. Both the genus and species names are gorilla; thus the binomial classification is Gorilla gorilla. The most common are the lowland gorillas, but there is also a smaller, rare mountain gorilla, considered to be a subspecies. Gorillas have marked sexual dimorphism, the size of mature males being ca. twice that of females. Next to chimpanzees, gorillas are the closest relatives of humans.

graft-versus-host disease   A condition in which bone marrow transplanted to another person mounts an immune response against the "foreign" antigens of the recipient.

great ape   Any of the large apes, i.e. orangutans, gorillas, and chimpanzees. The great apes are our closest relatives among the primates.

growth hormone   A protein hormone secreted by the pituitary gland that acts on many other tissues to stimulate growth.  The human growth hormone gene is located on chromosome 17.  Mutations that cause the growth hormone to be nonfunctional cause one form of dwarfism when homozygous.  More details can be found in OMIM.  Administration of GH from other sources during childhood increases the growth of such persons.

guanine   A purine that occurs in the nucleotides of DNA and RNA.  Commonly abbreviated G in DNA and RNA sequences.

haploid   [Greek haploos single, simple.] A term used to describe a single set of chromosomes. A haploid set of human chromosomes consists of 22 different autosomes and one sex chromosome, either an X or a Y. Mature germ cells are haploid, but all other cells in mammals are diploid.

haplotype   A group of closely-linked genes or genetic markers among which recombination is very low. Because of this, a specific combination of alleles within the group tends to remain together on the same chromosome through many generations.

Hardy-Weinberg equilibrium    The case in which the frequencies of genotypes in a population matches those predicted by the Hardy-Weinberg law based on allele frequencies in that population.

Hardy-Weinberg law    The application of the binomial expression to calculate probabilities of various genotype combinations based on allele frequencies in a random mating population. The binomial is written (p + q)2, where p and q are the frequencies of two alleles. If p + q = 1, which is the case if there are only 2 alleles, then (p + q)2 = 1. The expression can be expanded to p2 + 2pq + q2 = 1, where p2 is the frequency of persons homozygous for the allele represented by p, 2pq is the frequency of heterozygotes, and q2 is the frequency of homozygotes represented by q. For more than two alleles, the expression is (p + q + r + ... + z)2 = 1.

heavy chain   One of the two types of polypeptide chains found in antibodies. See antibody for more details.

helper T-cell   See T-lymphocyte.

hemizygous   [Greek hemi half + zygous.] Having only one copy of a gene, chromosome, or chromosome segment. Males are hemizygous for genes on the X chromosome, since they have only one X chromosome. Similarly, if there is deletion of part of one chromosome of a pair, the genes on the undeleted chromosome are described as hemizygous if they are in the region that corresponds to the deletion of the homologous chromosome.

hemoglobin   The major protein in red blood cells whose function is to bind oxygen in the lungs, transport it to other tissues, and release it for use by the tissues.  Hemoglobin (Hb) is a tetramer, consisting of four protein subunits, two identical alpha or alpha-like globins and two identical beta or beta-like globins.  Each subunit includes a complex non-protein molecule called hemin, which gives Hb (and red cells) the red color and which is the site of oxygen binding.  Hemoglobins have been highly instructive in understanding how genes function.  The analysis of sickle-cell Hb provided the first evidence in any organism that different alleles code for different amino acid sequences.  Members of the families of alpha-like and beta-like Hb genes are "turned on" at different stages of fetal and embryonic development, providing the opportunity to study how such switching occurs.  Many of the hundreds of mutant forms of Hb are quite detrimental but nevertheless occur at high frequencies in some populations.  These illustrate the complexities of natural selection in human and other populations.

hemolytic disease of newborn (HDN)   A condition in newborns due to production of antibodies by the mother against antigens on the child's red cells.  This can only happen if the antigen is absent from the mother's cells.  The most common occasion for HDN is when the fetus is Rh-positive and the mother is Rh-negative.  Typically the mother does not become immunized against the fetal cells until the second such incompatible pregnancy.  This is because the major introduction of fetal red cells into the maternal circulation occurs at the time of delivery.  The immune response can be usually prevented by injecting antibodies against the fetal red cells shortly after delivery, thus destroying them and removing them from circulation.  The next incompatible pregnancy is thereby spared.

hemophilia   A condition in which factors necessary for clotting of blood are deficient or lacking. The most common form is hemophilia A, which results from lack of coagulation factor VIII. The gene for this factor is on the X chromosome at Xq28. Mutations are recessive, with heterozygous females having normal blood clotting. Hemizygous males bruise easily, have bleeding into joints, and bleed profusely from small cuts. A less common form (hemophilia B, Christmas disease) is caused by lack of coagulation factor IX, the gene for which is also on the X chromosome. These two hemophilias are classic examples of X-linked recessive inheritance. Links to these disorders can be found at OMIM.

heritability   That part of the total population variation in a complex trait that is attributed to variation in genotypes. The remaining part of the total variation is environmental. Since both genotypic variation and environmental variation can differ among populations, the heritability measured on a particular population applies only to that population. Other populations may be more uniform or less uniform for the genes that contribute to the trait, and the same may be true of environment.

hermaphrodite   [Greek Ermafrodito, son of Hermes and Aphrodite, who, according to mythology, merged with the nymph Salmacis and thus acquired both male and female characteristics.]. An organism with both male and female genitalia. In modern medicine, a person with both ovarian and testicular tissue. Persons with only ovaries or testes but with some phenotypic traits of the other sex are pseudohermaphrodites.

heterodisomy   Uniparental disomy in which the two chromosomes are derived from the two homologs of the parent.  They should be different in the centromeric regions but can be identical in more distal segments due to crossing over in meiosis I.

heterogametic   Producing more than one type of gamete with respect to sex chromosomes. In mammals, males are heterogametic, producing both X-bearing and Y-bearing sperm. See homogametic.

heterozygous   The presence of different alleles at a specific locus on the two homologous chromosomes of a diploid organism.

histone   A basic protein associated with DNA in cells.  There are several different histones, which join together to form a structure around which DNA wraps ca. 1.75 times.  Each such DNA-histone structure is a nucleosome.  In a single nucleus, there is a vast number of nucleosomes.

holandric    [Greek hol- whole + andros man.] An older term for Y-linked, i.e. genes that are on the Y chromosome and therefore are transmitted only by males.

hominid   Any of the species, including humans, in the evolutionary lines after the separation from the common human/chimpanzee ancestor. These comprise the family Hominidae.

hominoid   [Latin homo man + Greek oeidos likeness.] A member of the superfamily Hominoidea, which consists of the small apes, the great apes, and humans.

Homo   [Latin homo man.] The genus that includes modern humans as well as early or archaic humans. The earliest fossils, from H. habilis, are from 2.4 million ybp (years before present) and extend to 1.6 million ybp. These all occur in east Africa. About 1.8 million ybp, a new fossil type, H. erectus, appeared and survived in Africa until some 400,000 ybp. H. erectus was the first hominid to move out of Africa, spreading widely in Asia where it survived to perhaps 200,000 ypb. About 400,000 ybp, the first members of our own species, H. sapiens, appeared in east Africa. These archaic forms spread out of Africa into Europe and the Middle East, giving rise to H. neanderthalensis, which survived until 30,000 ybp. Modern H. sapiens arose in Africa 100,000 to 200,000 ybp. Some moved out of Africa ca. 100,000 ybp, spreading to all other parts of the world.

homogametic   Producing a single type of gamete with respect to sex chromosomes. In mammals, females are homogametic, with all gametes having a single X chromosome. See heterogametic.

homologous   As commonly used in genetics, homologous describes the two members of a pair of chromosomes in a diploid organism. Members of a homologous pair have the same array of genes, although the gene pair may vary slightly in structure.

homozygous   The presence of identical (or similar) alleles at a specific genetic locus on the two homologous chromosomes of a diploid organism.

homunculus   The miniature individual that the theory of preformation proposed to exist in germ cells. The poor quality of early microscopes, coupled with the imagination of the observers, led to reports that homunculi were visible. However, they have not survived improved microscopes.

hormone   A substance secreted by an endocrine gland (e.g. pituitary, adrenal, gonads) into the blood stream and that regulates the activities of tissues and glands in other parts of the body.  Hormones typically are proteins, small peptides (8 to 12 amino acids), or steroids.

human immunodeficiency virus (HIV)   A retrovirus that preferentially attacks helper T-cells of humans, thus interfering with both humoral and cellular immune responses.

human leukocyte antigens (HLA)   Antigens of the major histocompatibility complex (MHC). In humans, these were first detected on leukocytes; hence their designation as HLA. They were subsequently shown to be homologous to the mouse H-2 histocompatibility complex, and the two systems are now referred to as MHC.

humoral immunity   An immune response that is based on the production of soluble antibodies in the blood plasma.

Huntington disease   A deterioration of certain central nervous system functions that lead to choreiform movements (involuntary jerky movements) and loss of mental abilities. Huntington disease (HD) is inherited as a dominant trait with late onset, usually in the fourth decade. The gene involved is located on chromosome 4p.  HD is an example of trinucleotide expansion as the usual form of mutation. Additional information and links can be found in OMIM.

hybrid   A cross or mating between two genetically distinct parents. The parents may differ by only a few genes, or they may belong to different but closely related species (interspecific cross) when such crosses are possible.

hydrocarbon   A chemical compound composed entirely of hydrogen and carbon atoms bound covalently. Examples are gasoline, methane, and butane. They are very hydrophobic and, generally being less dense than water, float on water without mixing. There are many possible ways that H and C atoms can be combined without violating the valence rules.

hydrogen   [Symbol H; atomic number = 1, atomic mass = 1.008.]  The smallest element.  There is one proton in the nucleus, and there are no neutrons in the common form (isotope), although forms with one or two neutrons exist.

hydrogen bond   A weak chemical attraction that occurs between the hydrogen of an amino group or a hydroxyl group and the unpaired electrons of another nitrogen or oxygen.  The two molecules of which these are a part must be in specific spatial orientations with each other.  Little energy is required to break hydrogen bonds: the temperature of boiling water is sufficient.  Indeed, the process of cooking an egg is primarily one of breaking hydrogen bonds, allowing the polypeptide chains to become randomly arranged rather than orderly.  Hydrogen bonds are crucial to DNA replication and transcription and to formation of functional proteins.

hydrophilic   [Greek hydor water + philos loving] Descriptive of substances that mix readily with water. Chemical structures that form ions or that are rich in oxygen generally are hydrophilic. Examples: sugar, salt.

hydrophobic   [Greek hydor water + phobos fear] Descriptive of substances that do not mix with water. Examples: fats, petroleum.

hydroxyl group   A functional group consisting of the structure –O–H.  It is usually connected covalently to carbon.  A hydroxyl group is hydrophilic and increases the water solubility of the molecule of which it is a part.  If that carbon is connected only to other carbons or to hydrogen, the hydroxyl group is often described as an alcohol.

hypercholesterolemia   The name means high levels of cholesterol in the blood. This description applies to many people, whose condition is due in part to diet and in part to heredity.  Some 1 in 500 persons is heterozygous for familial hypercholesterolemia, in which blood values for total cholesterol are 250-450 mg/dl. In the rare homozygotes, total cholesterol values are >500 mg/dl. Heterozygotes typically die of cardiovascular problems in middle age, whereas homozygotes die of heart defects as children. The problems are caused by defective function of the LDL (low density lipoprotein) receptor, which normally transports LDL-bound cholesterol from the blood into cells. The gene for the LDL receptor is on chromosome 19 at 19p13. Additional information and links are available in OMIM.

hyperekplexia   Excessive startle response. The genetic defect is in the glycine receptor-1 locus on chromosome 5. Additional information is available in OMIM.

hypertension   Elevated arterial blood pressure.

immune response   The defense system of the body that combats foreign substances, such as bacterial and viral infections.  There are two principal and related forms: humoral immunity and cellular immunity.  Humoral immunity consists of antibodies in the blood plasma that bind to specific foreign (nonself) antigens, causing them to be removed.  Antibodies are produced by B-cells. Cellular immunity consists in attack and binding by T-cells.  Both systems depend on specific recognition of foreign antigens, and both are dependent on DNA rearrangement to generate the variety of antibodies and T-cell receptors necessary to achieve the high level of specificity for diverse antigens.

immune tolerance   The failure to form antibodies against ones own tissue antigens. The distinction between self and nonself occurs in late fetal development and involves deletion of all B- and T-cells that are specific for self.

immunoglobulin (Ig)   A term that is equivalent to antibody.

imprinting   As used in genetics, imprinting refers to temporary inactivation of certain genes depending on the sex of the parent from which they are received. For example, a particular allele may become active in an offspring only if it is transmitted by the mother. The normal paternal allele will not be active in the offspring. Therefore, if the maternal allele is inactive because of mutation, neither will be functional in the offspring. Imprinting is also a consideration in uniparental disomy. If both chromosomes come from the same parent and happen to have an imprinted region, then the genes in that region are inactive on both chromosomes. The imprinting is reset each generation. Examples in humans are Prader-Willi syndrome and Angelman syndrome. There are few identified imprinted regions in humans, and most chromosomes are known not to have them. Since some genes are imprinted in the mother and other loci are imprinted in the father, an offspring can develop normally only if if has a mother and a father. See parthenogenesis, hydatidiform mole.

in vitro fertilization (IVF)   Although the name literally means "fertilization in glass" (in a test tube), it is actually done in plastic containers. "In vitro" is a time-honored phrase meaning to do things in the laboratory. In this instance, the egg donor is induced by hormones to superovulate (release multiple ova). The ova are collected surgically and are mixed with donor sperm. Those that are observed microscopically to be developing normally are then suitable for transfer into the female who is to serve as the gestational mother (often but not always the biological mother). There are several reasons for use of IVF, including diagnosis of genetic disorders in the preimplantation embryo.

incomplete dominance   This term is applied to alleles that are expressed in heterozygous combination with other alleles, but the heterozygous phenotype is different from that of homozygotes for the allele.

independent assortment   One of Mendel's major findings that parental combinations of alleles at different loci are not maintained during formation of gametes. With the discovery that chromosomes are the basis for the physical transmission of genetic traits, it became apparent that whether a chromosome is maternally or paternally derived has no bearing on its distribution to daughter cells in meiosis. In addition, crossing over generates new chromosomes with combinations of maternally and paternally derived alleles. An exception to independent assortment occurs if two genes are very near each other on a chromosome, since the frequency of crossing over between them is low.

initiation codon   The codon that is used to initiate all translation.  This codon, AUG in RNA codes and ATG in DNA codes, codes for the amino acid methionine.  This means that at the time of translation, all polypeptide chains begin with methionine at the N-terminus.  However, in most cases, the methionine is removed shortly thereafter and rarely occurs in completed proteins.  (See genetic code.)

inner cell mass   The mass of cells in a very early embryo from which the embryo proper develops. The remaining cells form the birth membranes.

inorganic molecule   Any molecule that does not include carbon in its composition.

insertion   Insertion of one or more nucleotides into DNA at a specific point.  If this happens within the coding region of a gene, the reading frame may be altered.

intelligence quotient   Perhaps better known as IQ, the intelligence quotient is derived by administering a test of mental achievement that has been standardized for each age group and then dividing the score by the chronological age of the subject (and then multiplying by 100 to get rid of fractions.) A child who performs at the exact average for his or her age would have an IQ of 100. Those performing at a more advanced level than the chronological age would have scores greater than 100. The fact that you are reading this means that your IQ is probably greater than 100! There has been much argument about the nature of intelligence and whether the tests used actually measure intelligence. They do seem to measure something of which intelligence is a component and to be somewhat predictive of academic ability.

interphase   The part of the nuclear cycle following the end of one division to the beginning of the next. Interphase can be divided into three parts: G1, in which the DNA has yet to replicate; S, the period in which DNA replication occurs; and G2, the period between S and the beginning of mitosis or meiosis.

interstitial cells   In general, cells located between other structures. Used especially to refer to the cells in the testis that are in the space between the seminiferous tubules. The interstitial cells are the site of testosterone synthesis in the testis.

intron   A segment of DNA that lies within the coding region of a gene but that does not code for amino acid sequences.  These segments, which are transcribed and therefore are in the primary transcript, are removed in the nucleus by a process called splicing.  When the introns are removed, the remaining segments (exons) are joined together to form messenger RNA.

inversion   A chromosomal rearrangement in which there are two breaks in a chromosome, with inversion of the central segment prior to chromosome repair.  Usually there would be no loss of genes, and therefore no effect on the phenotype.  However, inversions generate problems in meiosis, since specific chromosome pairing can occur only by formation of an inversion loop.  Crossing over within the inversion loop can lead to unbalanced gametes.

inversion loop   The loop that results when homologous chromosomes, one with an inversion, pair.

ion   An electrically charged atom or molecule.  The charge arises from loss or gain of electrons in the outer electron shell of an atom.  In the case of loss of one or more electrons, which are negatively charged, the remaining atom becomes positively charged because of the positively charged protons in the nucleus.  In the case of gain of one or more electrons, the atom becomes negatively charged.  A positively charged atom or molecule is called a cation, and a negatively charged atom or molecule is an anion.

ionic bond   A chemical bond that consists of the attraction between two ions of opposite charge.  Ions of the same charge, either positive or negative, repel each other.  Ionic bonds are not rigid and allow for frequent exchange of partners or simultaneous attraction to two other ions.  A parallel would be a dance hall filled with equal numbers of heterosexual males and females.  There may be frequent exchange of partners, but pairing would occur, at least for short periods.

isodisomy   In the case of uniparental disomy, both chromosomes having the same centromeric and adjacent regions.  In complete isodisomy, the two homologous chromosomes would be entirely identical and therefore homozygous for all loci.  This would occur if the zygote were originally monosomic, with duplication of the single chromosome to restore the euploid state.  If the two homologous chromosomes were due to nondisjunction at meiosis II, only the regions near the centromere would be identical because of crossing over in prophase I.

isogenic   [Greek isos equal + -genic.] Having identical genotypes. The term is used especially in genetics to refer to strains of organisms, all members of which are genetically identical (except for sex chromosome differences). In mice, for example, many generations of brother-sister mating has produced a number of strains in which virtually all genetic variability has been eliminated within each strain. Such strains are useful in certain testing procedures where the confounding effect of genetic variability is to be avoided.

isotope   A variation of an element that has the same atomic number (i.e. the same number of protons in the nucleus) but a different number of neutrons.  For example, hydrogen may have [1p,0n], [1p,1n], or [1p,2n] nuclei. Since there is no difference in the number of protons (and hence, electrons), all have identical chemical properties and are the same element.  However, the masses differ because of the neutrons.  The ratio of protons to neutrons strongly influences the stability of the nucleus.  In the case of hydrogen, nuclei with either 0 or 1 neutron are very stable.  However, nuclei with 2 neutrons are unstable and disintegrate spontaneously, releasing energy in the form of radiation.  This isotope of hydrogen is therefore a radioactive isotope.  Most other elements are similar, having one or more stable isotopes plus one or more unstable isotopes.  In the case of elements with high atomic numbers, none of the isotopes may be stable.

karyo-, -karyon [Greek karyon, kernel.] Prefix and suffix referring to the cell nucleus. Examples: karyotype, heterokaryon.

karyotype A standardized array of chromosomes. Typically, chromosomes are cut out from a photograph of a metaphase spread and are then pasted into the appropriate position, starting with the two chromosomes 1, followed by chromosomes 2, etc. Karyotypes are very useful in evaluating the chromosome status of a person or of tissue samples and cell cultures.

karyotype formula   A method for designating the karyotype of a person.  The formula consists of the number of chromosomes, the complement of sex chromosomes, any autosomes that are deficient or in excess, and a description of any rearranged chromosomes.  A normal female is 46,XX and a normal male is 46,XY.  A female with trisomy 21 is 47,XX,+21.  A male with Klinefelter syndrome is 47,XXY.  A male embryo that is monosomic for chromosome 5 is 45,XY,-5.  The description of rearranged chromosomes is more complex.

kingdom   The highest level of biological classification. See binomial nomenclature.

Klinefelter syndrome   A condition in males due to having two X chromosomes plus one Y chromosome.  The karyotype formula is 47,XXY.  Affected persons are tall, somewhat eunuchoid in appearance, and sterile.  They often have some breast development (gynecomastia).  Intelligence is normal.  The cause is nondisjunction involving sex chromosomes, which can lead either to an ovum with two X chromosomes or a sperm with one X and one Y chromosome.  There is increased risk with advanced age of the mother, consistent with many of the cases resulting from maternal nondisjunction.

knockout mouse   A mouse in which a specific gene has been inactivated. This is accomplished by introducing a nonfunctional gene by homologous recombination into embryonic stem cells in culture, injecting the modified stem cells into an early mouse embryo (blastocyst), and implanting the resultant chimeric embryo into a foster mother. Chimeric offspring produce nonchimeric gametes, some of which have the inactivated target gene. These gametes produce heterozygous mice, which can be interbred to produce homozygous mice. Knockout mice are useful in studying the function of specific genes by observing the effect on the phenotype when no functional form of the gene is present.

lactase persistence   The persistence beyond infancy of the enzyme lactase, necessary for the breakdown of lactose, the sugar in milk. The young of mammals, including humans, produce this enzyme through the nursing period. After that, its production diminishes in most humans and in other mammals. Persistence of lactase production is a dominant monofactorial polymorphic trait in all populations. Persons who continue to produce lactase can consume fresh milk with no problem. Persons without lactase often have difficulty with milk products because of the high level of lactose in the gut and the joy with which bacteria greet this condition. Diarrhea and other intestinal upsets follow. In Asian populations, the frequency of the persistent phenotype is very low; in Scandinavians, it is very high. Most of the world is inbetween. Additional information is available in OMIM.

lactose   The sugar that occurs in milk. Lactose is a disaccharide with one molecule each of glucose and galactose joined by a covalent bond. When consumed, it is broken down by the enzyme lactase into glucose and galactose, which are efficiently absorbed in the intestine. Many persons do not have this enzyme as adults and do not tolerate lactose in the diet. (See lactase persistence.)

LDL   See low-density lipoprotein.

LDL receptor   The plasma-membrane bound protein receptor that binds low-density lipoprotein (LDL).  LDL, which occurs in plasma, is a complex of cholesterol and proteins.  The receptor-LDL complexes are taken into the cell by endocytosis, the internalized particle fuses with a lysosome, and the cholesterol is released into the cell.  The LDL receptors are also released and recycle back to the plasma membrane.  Defects in LDL receptors are responsible for familial hypercholesterolemia.

lemur   An early primate, represented today by a number of species on the island of Malagasy. Lemurs are arboreal and largely nocturnal. The line that gave rise to lemurs and lorises split off from the line leading to humans some 55 million years ago.

Lesch-Nyhan syndrome   An X-linked recessive disorder that occurs as a result of lack of the enzyme hypoxanthine phosphoriboxyl transferase. Affected individuals are males who are mentally retarded. In addition they are prone to self mutilation, biting off lips, fingers, etc. Additional information is available in OMIM.

leukocyte   [Greek leukos white + kytos cell]  A white blood cell.  There are several types of leukocytes, all of which are involved in immunity.

light chain   One of the two types of polypeptide chains found in antibodies. See antibody for more details.

linkage Refers to genetic markers that are in the same linkage group.

linkage group A group of genetic markers that have been shown by genetic mapping to be close to each other on the same chromosome.

Linnaeus   The Latinized name of Carl von Linné (1707-1778), a Swedish botanist who established the binomial system of classification of organisms.

local population   A small population, typically a population that is reproductively isolated, that is genetically distinct. Roughly synonymous with local race.

local race   A small population that is genetically distinct from surrounding populations.

locus [Plural loci.] The location of a gene or other genetic variation or landmark.

loris   Primitive primates that share many features with their close relatives, the lemurs. The two groups separated from each other some 15 million years ago.

low-density lipoprotein (LDL)   A complex of cholesterol and proteins that occurs in plasma.  In this form, cholesterol is delivered to various tissues but primarily the liver, where it is taken into the cell.  The cholesterol released in the cell regulates further synthesis of cholesterol through a negative feedback system.  High levels of LDL in the blood are associated with high risk of heart disease.

lymphatic system   [Greek lympha water]  The system of vessels that drain excess fluid (lymph) from tissues, through collecting centers (lymph nodes), and eventually back into the blood stream.  Lymph nodes are important in immunity as collecting sites for bacteria and viruses and as the sites for production of lymphocytes.

lymphocyte [Latin lympha water + -cyte.]   A type of white blood cell. Lymphocytes are divided into B-cells and T-cells, both critical to immune responsiveness. Each of these types has subtypes with specific roles in immune response. Lymphocytes can be cultured in the laboratory and are the most common cell type used in characterization of chromosomes (karyotyping).

Lyon hypothesis   The hypothesis, first clearly stated by Mary F. Lyon, that only one X chromosome is active in mammalian cells with two or more X chromosomes. See X-chromosome inactivation.

lyse   [Greek lysis dissolution] Disruption of a cell membrane, destroying the integrity of the cell structure.

lysosome [Greek lysis dissolution + soma body]   An organelle whose function is the break down of various substances in the cytoplasm.

macrophage   A white blood cell of the granulocyte series that consumes foreign substances, especially those that have been marked by binding of antibodies.  Macrophages are the scavengers of our bodies, cleaning up the unwanted debris.

major histocompatibility complex (MHC)   The highly polymorphic complex of tissue antigens coded by a cluster of closely linked genes on human chromosome 6.  These antigens are primarily responsible for rejection of tissue and organ transplants that occurs when the recipient generates an immune response against "foreign" antigens.  (The prefix histo- refers to tissue.)  There are additional, minor histocompatibility loci, about which little is known since they do not usually cause problems.  The number of MHC loci and the high degree of polymorphism at each means that the likelihood of a complete match of antigens between two persons selected at random is vanishingly small.  Yet that is an important consideration in organ transplants.  A mismatched transplant is likely to be rejected because of an immune response on the part of the recipient.  Because of the proximity of the MHC loci to each other, crossing over within the complex occurs rarely, and the transmission of the allelic combinations (haplotypes) can be treated as a simple Mendelian system.  Parental crosses would typically be ab × cd, potentially producing children of four haplotype combinations: ac, ad, bc, and bd.  Thus a pair of siblings would have a 25% chance of matching, a prospect that is relevant to kidney and bone marrow transplants but not to most other organs.  The availability of drugs that suppress the immune system has made some degree of mismatch feasible, and partially incompatible organ transplants are now done frequently.

malaria   A disease due to infections by certain species of the genus Plasmodium. Symptoms include high fever and shaking chills, anemia, and enlarged spleen. Severe forms can lead to death. The infection is transmitted by Anopheles mosquitoes. Falciparum malaria, caused by P. falciparum, is the most severe form and occurs widely in tropical regions. Vivax malaria is caused by P. vivax and is less severe than falciparum malaria. It occurs in both tropical and temperate zones. Other species of Plasmodium can also cause malaria but are much less widespread. Malaria is thought to have been an important selective agent in human evolution.

malignant tumor   A tumor that is cancerous. Malignant tumors spread to other parts of the body and invade other tissues.

mammal   Members of the Class Mammalia. The distinguishing feature of mammals is the production of milk to nurse the young.

map unit   A measure of the distance between genetic markers, as measured by recombination frequency. One map unit = 1% recombination = 1 centimorgan. See genetic map.

Marfan syndrome   A dominantly inherited disorder of the fibrillin-1 protein, which occurs in connective tissue. Affected persons are tall, with frequent spinal curvature (scoliosis) and developmental abnormalities of eyes and heart. The fibrillin-1 gene is located on chromosome 15 at 15q21.1. Additional information is in OMIM.

marker   See genetic marker.

maternal inheritance   Transmission of an inherited trait only from females and to all offspring of a female. Maternally inherited traits are due to variations in DNA of mitochondria (and to chloroplasts in plants). These cytoplasmic organelles are transmitted to the zygote only via the maternal gamete.

meiosis [Greek meiosis diminution.]   The formation of haploid germ cells from a diploid parent cell. Meiosis involves two steps. In the first, called meiosis I or MI, paired homologous chromosomes separate, reducing the chromosome complement in the daughter cells to haploid. Each of the chromosomes consists of two sister chromatids with an undivided centromere. An additional division then occurs (meiosis II or MII), which is comparable to mitosis in that the centromeres divide and sister chromosomes go to different daughter cells.

meiosis I   The first of two divisions in meiosis, often abbreviated MI. In prophase (prophase I), homologous chromosomes pair, and crossing over occurs. At metaphase (metaphase I), the paired homologues align to form the metaphase plate. At anaphase (anaphase I), the members of a homologous pair separate and move to opposite poles. The two daughter cells thus have a haploid set of chromosomes, each of which has two chromatids and an undivided centromere. MI begins with one diploid cell and ends with two haploid cells.

meiosis II   The second of two divisions in meiosis, often abbreviated MII. It is similar in many respects to mitosis. After the alignment of the chromosomes at metaphase (metaphase II), the centromeres divide, and the new sister chromosomes move to opposite poles during anaphase (anaphase II). MII begins with two haploid cells and ends with four haploid cells.

memory cell   A B- or T-lymphocyte that is long lived and that can produce specific antibodies or T-cell receptors. These cells arise after antigenic stimulation and serve as the "historians" of the immune system, providing a quick start in the event of subsequent exposure of the organism to the same antigen.

-mere [Greek meros part.]   A common suffix that means part, as in centromere.

messenger RNA (mRNA)   The form of RNA that is generated by transcription of a gene in the nucleus and that then moves to the cytoplasm where it binds to ribosomes for assembly of a protein. Often the primary transcript must be processed by splicing out the introns and modifying both the 5' and 3' regions in order to produce functional mRNA

meta- [Greek meta after, beyond.]   A prefix that is often used to mean outside of or parallel to. In genetics, it typically has a meaning equivalent to middle.

metabolic error   A defect in an enzyme that catalyzes a metabolic step, causing the metabolic conversion not to occur. The common cause of metabolic errors is a defective gene that codes for a nonfunctional or missing enzyme.

metabolic pathway   A sequence of consecutive metabolic steps where one of the products of one reaction becomes a substrate for the next.

metabolism   A general term referring to the chemical conversions that occur in an organism. Each conversion can be thought of as a step, and the vast majority of steps are catalyzed by enzymes. Enzymes, which are proteins, are coded by genes. Therefore, each metabolic step is catalyzed by an enzyme that consists of one or more different polypeptides. Each polypeptide in the enzyme corresponds to a unique gene.

metacentric   Descriptive of a chromosome in which the centromere is located approximately in the middle, creating arms of similar, though not usually identical, length.

metaphase   The phase of mitosis or meiosis in which chromosomes are maximally condensed and are aligned in a plane between the poles of the spindle. Metaphase marks the end of prophase. It is followed by anaphase.

metaphase plate   The array of chromosomes in a plane that characterizes metaphase. The metaphase plate forms between the poles of the spindle and perpendicular to the spindle axis.

metastasis   The spread of a malignant tumor to other parts of the body.

metric system   The measure of length used in science and most parts of the civilized world other than the U.S. One meter (m) = 39.37 inches. One centimeter (cm) = .01 meters = 2.54 inches. One millimeter (mm) = .001 meters. One micrometer = 10–6 meters. One nanometer (nm) = 10–9 meters.

microsatellite   Repeated DNA sequences (VNTR's) in which the repeated unit is 2-4 nucleotides. The variation in repeat numbers is a very useful genetic marker. Because of the small size of the repeats, the entire segment can be easily amplified by the polymerase chain reaction.

microtubule   A structural element that participates in many cell functions. Microtubules consist of protein subunits that associate under certain cell conditions and that dissociate under others. This allows microtubules to form and disappear rapidly. One of the numerous functions of microtubules is formation of the spindle during mitosis and meiosis. During metaphase, some of the microtubules attach to the centromeric regions of chromosomes, and the chromosomes are then pulled along the microtubules to the poles of the spindle during anaphase.

minisatellite   Repeated DNA sequences (VNTR's) in which the repeated unit is ca. 200 nucleotides long. The variations in repeat number generates restriction fragments of various lengths, making them very useful as genetic markers.

missense mutation   A nucleotide substitution in a codon that causes a different amino acid to be incorporated at the corresponding position in the protein.

mitochondrion   [plural mitochondria]   An organelle in the cytoplasm of eukaryotic cells that uses oxygen to oxidize simple organic substances, using the released energy to form ATP, the "energy currency" of biological systems. Mitochondria are unusual in that they contain small amounts of DNA that code for some of the gene products necessary for mitochondrial function. Many others are coded in the nucleus, however. Mitochondria replicate by division. Each has multiple copies of the DNA, and there are many mitochondria per cell, sometimes thousands. As a result, when a cell divides and each daughter cell receives a portion of the cytoplasm, there is a good supply of mitochondria in each. In sexual reproduction in many organisms, including mammals, the fertilized egg receives all its cytoplasm, including its mitochondria, from the ovum and none from the sperm. Therefore, variations in mitochondrial DNA are maternally inherited, i.e. a mother transmits the variation to all children, and males never transmit the variation. A small number of human conditions are due to mitochondrial variations and show maternal inheritance.

mitosis   Refers to cell division in which the two daughter cells each have a genetic makeup that is identical to that of the parent cell.

MNSs blood groups   An antigenic system expressed on the surface of human red blood cells. The antigenic variations were first detected with rabbit antibodies produced by immunization with human blood cells. Eventually, the responsible antigens were identified as glycophorin A (MN groups) and glycophorin B (Ss groups), both proteins having strong structural homology in the N-terminal region and coded by two closely linked genes on chromosome 4. The MNSs blood groups are universally polymorphic in populations around the world. The MN variations (glycophorin A) were among the very few genetic markers available prior to 1955, and they were extensively used in genetic and anthropological studies for that reason.

molecular weight   The sum of the mass of all the atoms in a molecule.

molecule   A structure generated by two or more atoms held together by chemical bonds.

monoclonal   Cells or individuals that are descended from a single cell. The meaning is the same as clonal, but the prefix emphasizes the single cell origin. The term is used especially with certain antibodies and with tumors.

monofactorial inheritance   A trait whose variation is due to alleles at a single locus.

monohybrid cross   A mating, especially in experimental genetics, between parental stocks that have different alleles at one locus of interest. Other genetic differences between the parental stocks may exist but are ignored.

monomorphic   In genetics, a locus for which there is only one common allele. Common is defined arbitrarily as 1% or greater.

monosomy   The condition in a diploid organism of having only one of a particular chromosome rather than the expected two homologous chromosomes.  Monosomy occurs as a result of nondisjunction or of chromosome lag (failure of a chromosome to be incorporated into the daughter cell complement).  In humans, embryos that are monosomic for an autosome do not survive. Females who have only one X chromosome have Turner syndrome.

monozygotic   The term applied to twins who are derived from a single zygote. They are genetically identical and are also called identical twins or MZ twins. They arise when a very early embryo splits, and each of the two parts develops into a complete embryo. MZ twins do not appear to cluster in families. Therefore the tendency for MZ twinning does not appear to be inherited.

mosaic   An individual, some of whose tissues differ from the remainder by a single gene or chromosome mutation.  This arises when the mutation or nondisjunction occurs in a somatic cell during embryonic development.  Contrast with chimera. Mosaicism in gene expression can also result from X-chromosome inactivation, in which genes on only one of the two chromosomes is expressed.

Mullerian duct   In female embryos, the Mullerian ducts give rise to the fallopian tubes, the uterus, and the upper third of the vagina. In male embryos, development of these structures is prevented by anti-Mullerian hormone.

Mullerian-inhibiting hormone See anti-Mullerian hormone.

multifactorial inheritance   A trait whose variation is attributed to combinations of alleles at multiple loci and that may also be influenced by environmental factors.

multiple alleles   The existence of two or more alleles at a particular locus. Some loci have only one common allele and are called monomorphic. Others have two or more common alleles and are polymorphic. Every gene locus has the potential to mutate to a vast number of different forms. For certain loci that have been extensively examined, such as the globin loci of hemoglobin, hundreds of variant alleles have been recorded, most of which are rare. The phenotypic effects of many alleles are likely to be indistinguishable. For example, many will cause the protein product of the locus to be completely inactive, leading to the same phenotype. In other instances, subtle phenotypic differences may be associated with different alleles.

multiple polyposis of the colon   Several dominantly inherited conditions in which there are many, sometimes hundreds, of benign tumors (polyps) in the colon and intestine. The probability is high that one or more of the polyps will become malignant. Although the cancer per se is not inherited, pedigrees in which a multiple polyposis allele is segregating show typical Mendelian dominant patterns for colon cancer.

muscular dystrophy   Deterioration of muscle functions. The most common form is due to mutations at the dystrophin locus on Xp. Some mutations produce severe effects (Duchenne muscular dystrophy); others are milder (Becker muscular dystrophy). Both, however, are due to mutations, usually deletions, in the dystrophin gene. Duchenne MD typically appears in young boys, with progressive deterioration and death in the teens. Duchenne and Becker MD are classic examples of X-linked recessive inheritance. Additional information and links are at OMIM.

mutation   A heritable change in DNA structure.  Mutations may modify the coding region of a gene, the regulatory region, or both; or they may be limited to intergenic regions.  They may be as small as single nucleotide changes or as large as deletions of blocks of genes.

mutation rate   The rate at which mutations occur, either spontaneously or induced by exposure to external agents.  Mutation rates can be expressed in various ways, depending on the means of observation, the tissue, and the use to be made of them.  Most commonly in human genetics, the mutation rate µ = mutations per locus per gamete (or per generation, which is equivalent).  Mutation rates based on alteration of phenotype do not include those changes in DNA that do not affect the phenotype.  Phenotypic changes also can occur as a results of mutations in noncoding regulatory regions of genes.  Therefore every mutation rate should include the endpoint used for detection.  Typical mutation rates for rare dominantly inherited diseases are in the range 10–4 to 10–6, with an average about 10–5 mutations per gamete per locus.  This means that a particular sperm or ovum has an average chance of 1 per 100,000 of having a detectable mutation at the locus in question.  Mutation rates can differ between males and females.  The highet mutation rate known is for the Duchenne muscular dystrophy (DMD) gene—ca. 1 per 10,000 gametes.  This is very likely due to the very large size of the DMD gene—2.5 million nucleotide pairs—that presents a very large target for mutation.  The mutation rates that have been measured may be unrepresentative of all genes, since genes with very low mutation rates are less likely to be known and selected for study.  A figure of 10–6 is perhaps more representative.  Mutation rates for single nucleotide changes, irrespective of phenotypic effect, have been estimated about 10–8/nucleotide/gamete.

N-terminus   The end of a polypeptide chain at which is found the free amino group. In assembly of polypeptide chains in the cell, the process starts with the N-terminal amino acid. Its carboxyl group is joined by a peptide bond to the amino group of a second amino acid, and so on. The amino group on the first amino acid points away from the growing end of the polypeptide chain and never forms a peptide bond.

natural killer cell   A type of leukocyte that attacks and destroys other cells, especially virus-infected cells and some tumor cells.

natural selection The differential reproduction (fitness) of organisms whose gene combinations provide for better adaptation to their environment. Natural selection was proposed independently by Charles Darwin and Alfred Russel Wallace in 1858 as the basis for evolution.

neoteny   [Greek neos new + teinein to extend.] The retention in adults of childhood characteristics.

neural tube defect  A congenital defect of the spine in which the embryonic structure known as the neural tube fails to close properly. This may result in bulging out of the spinal cord (spina bifida) and in other abnormalities such as anencephaly.

neutron   A subatomic particle that has the mass of a proton but that is electrically neutral.  Variation in the number of neutrons in an atomic nucleus changes the mass of the atom but not the chemical properties.

New World monkeys   Monkeys of the superfamily Ceboidea, found at present in South and Central America. Generally rather small, many have developed prehensile tails that can be used to hang from branches or grasping objects. The Ceboidea are thought to have diverged from the primate lineage leading to humans earlier than the Old World monkeys.

nitrogen   [Symbol N; atomic number = 7; atomic mass = 14.007.]   One of the elements that plays an important role in biological systems.  One atom of nitrogen joins with two atoms of hydrogen to form an amino group, one of the important functional groups in amino acids and other biological molecules.

nondisjunction   The failure of chromosomes to separate properly in mitosis or meiosis.  Rather than the two homologous chromosomes moving to opposite poles of the spindle, both move to the same pole.  This causes one daughter cell to have an extra chromosome and the other to have a deficiency.  Ordinarily, a single chromosome pair is involved in nondisjunction, but rarely two or more be involved.  Meiotic nondisjunction is responsible for several instances of chromosome imbalance, involving especially chromosomes 13, 18, and 21 and the sex chromosomes.  In fact, nondisjunction occurs with all chromosomes, but only those listed are observed in newborns.  The remainder routinely abort spontaneously.  Meiotic nondisjunction occurs primarily in meiosis I of females, and the risk increases strongly with age.

nonrecombinant The presence in a gamete of alleles that were received by the producer of the gamete from the same parent.

nonsense mutation   A nucleotide substitution that generates a termination (stop) codon in the coding region of a gene instead of the amino acid specified by the original codon.  During translation, assembly of the polypeptide chain will terminate at that codon, and the resulting protein will be missing the usual C-terminal amino acid sequence.

nuclear envelope Sometimes called the nuclear membrane, it is actually a double membrane that encloses the nucleus of eukaryotic cells. The nuclear envelope has many nuclear pores that regulate the passage of substances between the nucleus and cytoplasm.

nucleic acid   Complex molecules that are formed by assembly of nucleotides into a linear polymer.  Two types are found in biological systems: ribonucleic acid (RNA), in which the building blocks are ribonucleotides, and deoxyribonucleic acid (DNA), in which the building blocks are deoxyribonucleotides.  DNA is found almost exclusively in nuclei and is the primary repository of genetic information.  RNA is found both in cytoplasm and nuclei and is a secondary repository of genetic information (with the exception of some viruses, in which it is primary).  Nucleic acids can be described as informational molecules because of their ability to store information in the form of different nucleotide sequences.

nucleolus [Plural nucleoli] Darkly-staining bodies seen in nuclei. Nucleoli are the sites of ribosomal RNA synthesis and ribosome assembly prior to movement of the ribosomes to the cytoplasm.

nucleosome   Nuclear structures composed of histones around which DNA is wrapped.

nucleotide   A type of complex organic molecule that constitutes the building block of nucleic acids.  A nucleotide consists of three components: a pyrimidine or purine base, a 5-carbon sugar, and phosphoric acid.  The three are linked by covalent bonds and are often represented B-S-P.  In RNA, the purines are adenine and guanine; the pyrimidines are cytosine and uracil.  In DNA, the bases are the same except that thymine occurs in place of uracil.  In RNA, the sugar is ribose, hence the name ribonucleic acid.  In DNA, the closely related sugar deoxyribose occurs (deoxyribonucleic acid).

nucleotide analog   A molecule that is structurally very similar to a naturally occurring nucleotide. In some instances, DNA polymerase will incorporate the analog into a strand but cannot continue beyond that point. DNA sequencing methods exploit this property.

nucleotide substitution   The replacement of a nucleotide (A, G, C, or T) with any of the other three nucleotides at a particular position in DNA.  This is a common type of mutation.  If it occurs within the coding region of a gene, the codon will change, producing either a synonymous mutation or a missense mutation.

nucleus [plural nuclei] An organelle that occurs within eukaryotic cells. It contains the chromosomes and genes and, accordingly, is the master control center of the cell. It is encased in a double-membrane nuclear envelope that regulates the passage of materials between the nucleus and cytoplasm.

nullisomy   Absence of any copies of a chromosome.

Old World monkeys   Monkeys of the superfamily Cercopithecoidea, found in Asia and Africa. Prominent members are the macaques and the baboons. Many are arboreal, but some are entirely terrestrial. This diverse group of monkeys is thought to have diverged from the hominoid line after the New World monkeys.

oncogene   A gene whose overexpression is associated with cancer. Overexpression occurs by any of several mechanisms. Oncogenes were first identified by their presence in certain viruses. When it was recognized that the virus originally got its copy of the oncogene from the host and that the gene was normal in the host cell, the two forms of the oncogene were designated v-oncogene (viral-oncogene) and c-oncogene (cellular-oncogene, also known as proto-oncogene). The v-oncogene is under regulation of the viral promoter, and transcription occurs along with viral genes. Dozens of oncogenes have been identified, and they normally function in a number of different ways in cells. The common element is that overexpression interferes with the normal regulation of cell growth and leads to inappropriate growth and division.

oöcyte   A germ cell in the ovary that has initiated meiosis but not completed it. A cell that is in meiosis I is called a primary oöcyte; one that has completed meiosis I is a secondary oöcyte. In mammals, the ovum is actually a secondary oöcyte that completes meiosis only if fertilized.

oögenesis   The process of producing an ovum.

orangutan   One of the great apes, assigned to the genus Pongo, found today in Borneo and Sumatra. In spite of their size, orangutans are primarily arboreal and use brachiation as a means of locomotion. The orangutan evolutionary lineage separated from the gorilla/chimpanzee/human lineage some 15 million years ago, but there are still marked similarities between orangutan and human karyotypes.

order   The level of biological classification greater than families and less than classes. See binomial nomenclature.

organelle   Any of the structures that occur within a cell, such as nuclei, mitochondria, lysosomes, etc.

organic molecule   Molecules that include carbon in their composition.  Such molecules were once thought to form only in organisms; thus the name.  Proteins, carbohydrates, nucleic acids, fats, etc. are all organic molecules.

osteogenesis imperfecta   Any of several inherited disorders of collagen structure. A conspicuous feature is brittle bones, with multiple fractures following mild trauma. Access to entries on osteogenesis imperfecta can be found in OMIM.

ovarian follicle A structure in an ovary that contains a single oöcyte. Immature follicles contain oöcytes that are arrested in late prophase I. Once each month, a follicle matures and releases an ovum, which is a secondary oöcyte.

ovarian teratoma [Greek teratos monster + -oma tumor.] A tumor that forms in ovaries and that is comprised of various types of embryonal tissues. Genetic analysis indicates them to be derived from maternal germ cells through several mechanisms. They are diploid, but many are homozygous for loci near the centromere. This could occur if the chromosomes in a secondary oöcyte divided without cytokinesis to form a diploid cell. Without a male contribution, such a cell does not develop normally, presumably because of parental imprinting of genes.

ovary The female gonad. The female germ cells are in the ovary and initiate meiosis there. The ovary also secretes estrogens, promoting the development of secondary sexual characteristics.

ovum An egg. In mammals, the ovum is a secondary oöcyte. If fertilized, the nucleus of the oöcyte completes meiosis II, and the resulting haploid nucleus fuses with the male pronucleus introduced by the sperm.

oxygen   [Symbol O; atomic number = 8; atomic mass = 15.999.]   A very important element for biological systems and for other processes.  Many substances combine with oxygen, either directly or indirectly, releasing energy that can be used for other purposes.  In many organisms, including all eukaryotes, this process of oxidation causes energy to be transferred to ATP, which then serves as an energy source for many other processes.  Oxygen is an important component of a very large number of biological molecules.

P, F1, F2 Symbols used in experimental genetics to indicate the parental generation in a cross between two stocks, the offspring of that initial cross (F1, first filial generation), and the offspring produced by crossing the F1 individuals with each other (F2, second filial generation.).

p,q The chromosome arms, defined by the position of the centromere, typically are unequal in length. The short arm is designated p (petite), and the long arm is q (which follows p in the alphabet). For example, a gene located on 6q would be on the long arm of chromosome 6.

32P-label   Incorporation of the radioactive isotope of phosphorus into substances whose presence can then be detected by the radiation from the decaying 32P.

palindrome   A sentence or phrase that reads the same backward or forward, e.g. "Madam, I'm Adam." In molecular biology, palindromes are complementary single-strand DNA sequences that are identical. For example, the sequence 5'-AGCT-3' would have as a complementary sequence 3'-TCGA-5', which is identical, though the two strands would pair in an antiparallel manner.

Pan   The genus into which chimpanzees are placed.

pangenesis The theory that all parts of an organism contribute to the information in a germ cell by means of particles called gemmules. This idea, formulated by Charles Darwin, is similar to theories of the ancient Greek philosophers. It is now known to be completely false.

panmixis    See random mating.

parthenogenesis [Greek parthenos virgin + genesis production.] Development of an ovum without fertilization by a sperm. In some species, parthenogenesis occurs by mechanical or chemical stimulation of the ova. The chromosome set may be doubled to produce a diploid embryo. Rumors to the contrary notwithstanding, there is no documented case of parthenogenesis in humans resulting in a live birth. Such an individual would, of course, be female in the absence of a Y chromosome. See, however, ovarian teratoma.

passive immunity   Immunity achieved by introduction of antibodies from another immune person rather than by active immunity. A common example are newborn children, who have antibodies from the maternal circulation. Injections of pooled gamma-globulin after exposure to an infectious agent is another example, based on the assumption/demonstration that the persons who contribute plasma to such a pool are likely to include some who have antibodies in their blood because of prior infection with the agent. Antibodies have an average life of only about three months, during which time they help protect the recipient. After that, there is no residual immunity.

PCR   See polymerase chain reaction.

pedigree analysis Examination of pedigrees, especially human pedigrees, to confirm or disprove that a particular trait occurs in families according to one of the patterns of transmission predicted by Mendelian theory. Pedigree analysis is especially useful in testing rare autosomal dominant traits and X-linked traits. It is not useful ordinarily in testing rare, apparently autosomal recessive traits, since affected persons would occur only in one sibship.

pedigree chart A graphical representation of a family, showing the relationships among family members and the phenotypes and genotypes of interest for persons in the family.

penetrance The likelihood that a particular genotype will be expressed in the phenotype. A penetrance of 100% means that the associated phenotype always occurs when the corresponding genotype is present.

peptide bond   The covalent bond that joins the carboxyl group of one amino acid to the amino group of another.

phenocopy An environmentally caused phenotype that mimics a genetic trait. Example: Phocomelia is a condition that involves major reduction of the limbs, such that hands and feet may be attached directly to the shoulders and hips, respectively. It can be caused either by homozygosity for an autosomal recessive gene (Roberts syndrome) or by exposure to the drug thalidomide during embryonic development.

phenotype A term that refers to the expressed traits of an organism, irrespective of whether or to what extent the traits are the result of genotype or environment.

phenotypic ratio The distribution of phenotypes among the offspring of a specific genetic cross. The phenotypic ratios may differ from genetic ratios depending on the dominant/recessive relationships of the alleles that are segregating.

phenylketonuria (PKU)   A recessive metabolic disorder in which the enzyme phenylalanine hydroxylase (PAH) is missing. This enzyme catalyzes the conversion of the amino acid phenylalanine to tyrosine. When PAH is missing, phenylalanine accummulates to high levels in the blood. It and some of the other products to which phenylalanine is converted interfere with other metabolic steps. The phenotype includes mental retardation, hyperactivity, and reduced pigmentation. Reduced phenylalanine in the diet of young affected children prevents development of these characteristics. The gene for PAH is on chromosome 12. Additional information is available in OMIM.

phosphoric acid   A chemical compound consisting of an atom of phosphorus, to which three hydroxyl groups and one double-bond oxygen atom are attached.  The hydrogen atoms of the hydroxyl groups are highly likely to dissociate as hydrogen ions, making phosphoric acid a strong acid.  The negative phosphate ion (PO4)3– occurs in biological systems associated with various positively-charged cations.  Phosphate can also form covalent bonds between one or more of its hydroxyl groups and hydroxyl groups on other molecules, splitting out water (H2O) in the process.  This occurs, for example, in nucleotides and in polynucleotide chains of DNA and RNA. Regulation of the activities of many proteins is by means of adding or removing phosphate.

phosphorus   [Symbol P; atomic number = 15; atomic mass = 30.974.]   A critical element in many biological processes.  Phosphorus usually occurs in biological molecules in combination with oxygen in the form of phosphate.

phylogeny   [Greek phylon tribe + genesi origin.]  The origins of the various groups of organisms; reconstruction of the evolutionary pathways that led to different organisms.

phylum   [pl. phyla] The level of biological classification greater than classes and less than kingdoms. See binomial nomenclature.

physical map A diagram or other representation that shows the physical location of genes or other genetic markers on chromosomes.

plasma cell   A large lymphocyte active in production of antibodies. See B-lymphocyte.

plasma membrane The membrane that encloses a cell.

plasmid   A DNA particle in some bacteria that has the ability to replicate with the bacterial cell but that is not part of the bacterial genome. Plasmids are important tools in genetic engineering, as they can be isolated, modified structurally, usually by insertion of genes or other DNA of interest, and inserted back into bacterial cells, where they replicate.

ploidy The ploidy of a cell refers to the number of sets of chromosomes it contains. In humans, a haploid cell has 23 chromosomes (22 autosomes + one sex chromosome). Diploid cells have 46 chromosomes, triploid cells have 69, etc. The ploidy is frequently represented by multiples of n, e.g. haploid cells have n chromosomes, diploid cells have 2n, triploid cells have 3n.

point mutation   A small mutation, often equated to change in a single nucleotide.  Also used, especially historically, to refer to mutations in a single gene in contrast to multigene deletions.  Prehistorically, any mutation that could not be observed by microscopic examination.

polar body At the completion of meiosis I in an oöcyte, one of the haploid products is extruded from the main body of the cell to form a polar body. It consists of little more than the extruded nucleus and a bit of cytoplasm, surrounded by a plasma membrane. If the ovum is fertilized and meiosis II is completed, one of the meiotic products is again extruded to form a second polar body. Polar bodies do not have a future.

polygenic A trait whose variation is due to a combination of alleles at multiple loci.

polymer   A large molecule that is produced by connecting subunits to form a chain.  In homopolymers, the subunits are identical, e.g. -A-A-A-A-.  In heteropolymers, two or more kinds of subunits are used, often in an alternating pattern, e.g. -A-B-A-B-A-B-.  These are examples of linear polymers, since assembly occurs only in one dimension.  Both DNA and RNA are linear polymers of nucleotides and can be either homo- or heteropolymers.

polymerase chain reaction (PCR)   A procedure for amplifying geometrically a particular segment of DNA. The principle depends on the requirement by DNA polymerase of a primer with a 3' end to which nucleotides can be added. Two such synthetic primers define a segment that is replicated in a thermal cycle of denaturation, reannealing (reformation of complementary primer-DNA structure), and replication. Each cycle, which takes 2-3 minutes, doubles the amount of DNA between the primer boundaries. Thirty cycles would thus yield 230 copies. PCR has made it possible to characterize extremely small amounts of DNA, such as that in a single sperm.

polymorphic In genetics, a locus at which there are two or more common alleles. Common is defined as having a frequency of 1% or greater.

polynucleotide   A polymer composed of nucleotides, either ribonucleotides or deoxyribonucleotides, joined by covalent bonds.  Let B-S-P represent a single nucleotide.  Both the sugar (S) and phosphate (P) can form an additional covalent bond.  When this occurs, S of one nucleotide forms a bond to P of the next; the S of the second nucleotide forms a bond to P of the third nucleotide, etc.  This generates a linear structure that can be represented -S-P-S-P-S-P-, with the purine and pyrimidine bases attached to the S groups and projecting out from the axis of the -S-P- chain.  Very large numbers of nucleotides can be assembled into a polynucleotide chain.  In the case of some human chromosomes, the DNA has tens of millions of deoxyribonucleotides in single, monstrous polynucleotide chains.

polypeptide   A chain of amino acids joined together by peptide bonds.

polyploidy   The condition of having three or more sets of the basic haploid complement of chromosomes.

Pongo   The genus in which orangutans are placed.

preformation The archaic idea that germ cells contain a completely formed individual. Some persons (the ovists) thought that the individual (homunculus) is in the ovum. Others (the spermists) believed it to be in the sperm. Preformation does not provide for equal contributions from both parents. It is the opposite of epigenesis.

prehensile tail   A tail that has the ability to "grasp" objects, such as tree branches. Prehensile tails are characteristic of many New World monkeys.

preimplantation embryo   An embryo from the time of fertilization of the ovum until the embryo implants into the uterine wall. Implantation occurs approximately 7 days after fertilization. The embryo at that time is a blastocyst.

premessenger RNA   Same as primary transcript.

premutation   An unstable "semimutation" that has no phenotypic effect but that is highly likely to mutate to a full mutation during transmission.  See trinucleotide expansion.

prenatal diagnosis   Establishing a diagnosis of an embryo or fetus before birth.  Used especially for identification of genotypes that are associated with major medical problems.

primary structure   Refers to the amino acid sequences of proteins.

primary transcript   Newly transcribed RNA prior to processing by splicing, etc.

primate   A member of the Order Primates, which includes humans, apes, monkeys, and the prosimians.

primer   The general meaning of primer is an object or act to get something started. As used in molecular genetics, it is a short polynucleotide chain that is necessary to initiate synthesis of a DNA strand by DNA polymerase. It is necessary because DNA polymerase catalyzes the addition of deoxyribonucleotides to the 3' end of a nucleotide that is bound to the template to be copied. The 3' end is provided by the primer.

pro- Prefix meaning before. Examples: prokaryote, before (the evolution of) karyotes; prophase, the phase before chromosome divisions.

proband The person whose phenotype led to recognition that a particular trait is present in a family; the first person in a family to be identified as having a trait and thereby calling attention to the family. Other terms with identical meaning are index case and proposit(us/a). The proband is usually indicated in a pedigree chart by an arrow. In some statistical analyses, it is important to know which family member is the proband.

probe   See DNA probe.

prokaryote [Also spelled procaryote.] A cell or organism without a nucleus. Examples include bacteria. All prokaryotes are one-cell organisms. They contain DNA, and the genes function much as in eukaryotes. However, the DNA is not organized into eukaryote-like chromosomes and is not contained within a membrane-bound organelle.

promoter   The part of the 5'-flanking region at which RNA polymerase binds to initiate transcription.

pronucleus The term used to describe the haploid female and male chromosome complements in a fertilized egg prior to their fusion to form a diploid nucleus.

prophase The initial phase of mitosis and meiosis in which the chromosomes condense and become visible and the nuclear envelope disappears. Prophase ends when the chromosomes align to form metaphase.

prosimian   [Latin pro before + simia ape.] The more primitive (from the human point of view) members of the Order Primates. The living prosimians consist of tree shrews, lorises, lemurs, and tarsiers.

protein   A biological linear polymer whose building blocks are amino acids. The amino acids are connected by a covalent bond between the carboxyl group of one and the amino group of the second. Such a dipeptide can add another amino acid to form a tripeptide, and so on. A polypeptide has larger numbers of amino acids in the peptide. A gene functions by coding for the sequence of amino acids in a peptide. Thus there is a direct relationship between the linear structure of DNA and the linear structure of polypeptide chains. Twenty amino acids are coded by DNA. Occasional modifications of one or more amino acids may occur after incorporation into a protein. A polypeptide chain is rarely functional as such. It must fold into a three-dimensioal form, the native form. See primary structure, secondary structure, and tertiary structure.

proton   A subatomic particle in the nuclei of atoms.  A proton has a single positive electrical charge.

proto-oncogene     See oncogene.

proximal   Refers to some event that is nearer to the centromere than some specified marker or landmark.

pseudoautosomal   Genes that are located in the small homologous segments at the tips of the short arms of the X and Y chromosomes. These pseudoautosomal regions pair during meiosis and undergo crossing over. When considered independently of other loci on the X and Y chromosomes, genes located in this region show transmission patterns characteristic of autosomal genes. However, they cosegregate with closely linked genes in the nonhomologous regions of the X and Y chromosomes.

pseudohermaphrodite   An individual with ovaries or testes but with characteristics of the opposite sex. An example is androgen insensitivity, in which a mutation in the androgen receptor gene makes 46,XY embryos with normal testes and testosterone production develop into phenotypic females.

Punnett square A convenient diagrammatic method for calculating probabilities of combinations of events, especially useful in genetic crosses. Attributed to R. C. Punnett, an early geneticist.

purine   Any of a series of  organic bases that have a double ring structure in common but that vary in the functional groups that are attached to the rings.  Nitrogen atoms in the rings are able to bind hydrogen ions, thereby causing the purines to be weakly basic.  The two purines that are of great importance in genetics are adenine and guanine, both of which occur in the nucleotides of DNA and RNA.

pyrimidine   Any of a series of organic bases that have in common a six-membered ring constructed of carbon and nitrogen atoms.  The pyrimidines vary in the functional groups attached to the ring.  The nitrogen atoms in the ring are able to bind hydrogen ions, causing the pyrimidines to be weakly basic.  The three pyrimidines that are of great importance in genetics are cytosine, thymine, and uracil, which occur in the nucleotides of DNA and RNA.

quaternary code   A code in which there are four choices or letters.  The binary code of computers, for example, has two choices, either 1 or 0.  In the DNA code, there are four choices, G, A, C, and T.  The number code commonly in use has 10 choices and is a decimal code.  The greater the number of choices, the fewer digits required to specify a given number, word, etc.

quantitative trait loci (QTL)   Genes whose variations contribute incrementally to a multifactorial trait.

race   A large population that is genetically distinct from other populations within the same species. The magnitude of the difference is not specified; hence, there can be quite varied opinions as to the existence of or number of races within a species.

radioactive isotope   An isotope that has an unstable nucleus and that spontaneously disintegrates, releasing energy in the form of radiation.

random mating    The selection of mates without regard to genotype. This is not as easy as it might seem. Mates are selected for a variety of attributes, many of which are influenced by heredity. Physical appearance is an obvious example. More subtle traits are intelligence and personality. In addition, we tend to select mates from our own social and ethnic groups, which may not be representative of the gene pool of the entire population. Random mating is not the same as haphazard mating. See assortative mating.

ras   An oncogene that was first identified in a rat sarcoma, hence the name. The normal function of the ras protein is the receipt of signals from receptors in the plasma membrane and transmission to other cell components and thence to the nucleus. Normal ras proteins, after having been turned on by the signal, have a mechanism for turning themselves off. Mutant forms of ras cannot turn themselves off and continue to send signals to the nucleus to divide, thus contributing to the abnormal growth of cancer cells.

reading frame   The reference frame of three nucleotides with which messenger RNA is "read" during translation. Translation is initiated by the mRNA sequence AUG.  That sets the reading frame, and the following codons are established by taking three nucleotides at a time.

receptor   A protein that binds to another substance (the ligand) in order to initiate or modify some function.  Many receptors are imbedded in the plasma membrane and bind to ligands outside the cell.  In some instances this generates a signal at the cytoplasmic end of the receptor that modifies cellular activities.  An example is the insulin receptor, which responds to the presence of insulin outside the cell.  In other instances, binding of the ligand to the receptor causes the complex to be taken into the cell.  The LDL (low-density lipoprotein) receptor is an example.  In the case of steroid receptors, the receptor is located in the cytoplasm.  Binding of estrogen or androgen to their respective receptors activates the receptors to move into the nucleus and modify the activities of specific target genes.

recessive Requiring two similar alleles in order to be expressed in the phenotype. Such individuals are homozygous for the recessive alleles.

reciprocal matings Matings in which, on the one hand, a particular genotype is in the paternal stock, and on the other hand, it is in the maternal stock. In Mendelian systems, there should be no difference in the F1 and F2 generations from reciprocal matings. This applies to the great majority of genes, but does not apply for genes on the X and Y chromosomes nor for imprinted genes.

recombinant DNA   DNA that has been assembled from DNA fragments of different origin. Often the recombinant DNA is a cloning vector into which one or more restriction fragments have been inserted.

recombinant plasmid   A plasmid into which DNA has been inserted.

recombination The combination in a gamete of alleles that were received by the producer of the gamete from different parents.

regression to the mean In complex inheritance, the mean values of offspring for a continuous trait tend toward the average of the parental values. This has misled some persons into thinking that phenotypic variation in a population should decrease each generation. This ignores the fact that the phenotypes of many offspring fall outside the parental range. Overall, the phenotypic variation in a population remains constant in succeeding generations, provided there is no change in environment or genotypes that modify the phenotypes.

repetitive DNA sequence   A DNA sequence that is repeated a number of times, sometimes in tandem, sometimes scattered throughout the genome. Approximately 30% of the human genome is repetitive DNA, with several sequences repeated hundreds of thousands of times. No function has been established for these sequences. Some of the sequences that are repeated lesser numbers of times are very useful as genetic markers. See minisatellite, microsatellite.

reproductive isolation   Lack of genetic exchange between populations. This can be due to geographic barriers, to behavioral barriers, or to reduced fitness of hybrids between the populations.

repulsion If two alleles of interest are on different members of a pair of homologous chromosomes, they are said to be in repulsion. If they are on the same chromosome, they are coupled.

restriction enzyme   An enzyme that catalyzes the cleavage of DNA at a specific nucleotide sequence. More properly called a restriction endonuclease. The more useful restriction enzymes cut DNA at palindromic sites, creating two identical ends, often with identical single-strand tails that are complementary to each other. For example, the target sequence 5'-AGTACT-3' has an identical complementary sequence 3'-TCATGA-5'. If both strands are cut between A and G, both would have single-strand tails 3'-CATG-5'. These tails, being complementary, tend to stick together, generating a double strand of DNA with gaps between A and G, which can be religated by formation of covalent bonds. DNA fragments from different sources, if cut with the same restriction enzyme, can be joined together by means of their sticky tails and ligation.

restriction fragment   A DNA fragment created by cleavage of a larger piece of DNA with a restriction enzyme. Very large pieces of DNA, such as the entire DNA in a genome, may be broken into many thousands of fragments. Each restriction enzyme generates its characteristic fragments because the DNA is cut only at target sites for that enzyme.

restriction fragment length polymorphism (RFLP)   Polymorphic variations in size of a DNA restriction fragment, useful as a genetic marker. There are two mechanisms for the origin RFLP's:  (1) A particular site on one chromosome will be cleaved by a restriction enzyme, whereas on another homologous chromosome it will not. A DNA probe that binds to either side of that site will indicate restriction fragments of different length, detectable in Southern blots. (2) The number of nucleotides between restriction sites varies. This is usually due to variations in numbers of repetitions of a nucleotide sequence. A Southern blot again will indicate different lengths for the restriction fragment. RFLP's are inherited as simple codominant Mendelian traits.

restriction site   A DNA sequence that is the target site of a restriction enzyme. Some target sites consist of as few as four nucleotide pairs. There would be many such sites in a large piece of DNA. Other restriction enzymes have targets with as many as eight nucleotide pairs and would cut DNA at many fewer sites.

retinoblastoma   [Gene symbol RB.] A malignant tumor of the retinoblast cells in the retina of the eyes of young children. The RB gene product is an important regulator of the G1 to S progression in the cell cycle. Cells that are homozygous for nonfunctional RB genes are unregulated. Persons who are heterozygous for RB are at high risk that at least one retinoblast cell will become homozygous through somatic mutation, thus generating a malignant tumor. Retinoblasts are incompletely differentiated embryonic cells present in young children. They disappear after several years. Retinoblastomas therefore occur only in young children, and the risk of retinoblastoma drops to near zero in older children. Additional information is available in OMIM.

retrovirus   A very small virus that stores genetic information in the form of RNA, which is copied into DNA for replication in an infected cell. The DNA copies are catalyzed by RNA-dependent DNA polymerase, commonly called reverse transcriptase. The DNA copies of the retrovirus integrate readily into the DNA of host chromosomes, where RNA copies are made by the transcription machineray of the cell. The ability to integrate into host DNA makes them candidates for use in gene therapy. Many retroviruses have acquired a copy of a host gene, probably from host messenger RNA. If the products of such genes stimulate cell growth and division, the result may contribute to cancer. The extra gene would then be an oncogene. There are a number of retroviruses, of which the most important to human health is the human immunodeficiency virus.

reverse transcriptase   The common term applied to RNA-dependent DNA polymerase. Making a DNA copy of RNA is the reverse of transcription. The enzyme is produced only by retroviruses, which use the cell machinery for replication. Since retroviruses store genetic information in the form of RNA rather than DNA, DNA copies must be made first in order for host DNA polymerase to act.

RFLP   See restriction fragment length polymorphism.

Rh (rhesus) blood groups   A system of polymorphic antigens on human red blood cells.  The antigens were originally discovered on the red cells of rhesus monkeys, hence the name rhesus blood groups, usually shortened to Rh.  The antigen was also independently discovered on red cells of a newborn suffering from hemolytic disease of the newborn.  Presence of the Rh antigen was shown to be a dominant trait, with Rh-negative persons homozygous for an allele associated with absence of the antigen.  The genetics of the Rh system is now known to be more complicated, but for many purposes it can still be treated as a two-allele locus.  People who are Rh-negative do not produce antibodies to Rh-positive antigens unless immunization has occured, either by blood transfusion of incompatible blood or by transfer of Rh-positive red cells from a fetus to an Rh-negative mother.  Blood to be used for transfusion is routinely matched to the recipient for Rh.  In reports of blood types, the Rh status is indicated by the "positive" or "negative" that follows the ABO type.  E.g. A person with type A, Rh-negative blood will commonly be described as type A, negative, which sounds to the novice as if A were negative.  But you are no longer a novice!

ribonucleic acid (RNA)   A polynucleotide composed of a chain of ribonucleotides.  The sugar in the nucleotides is ribose, and the bases are adenine, cytosine, guanine, or uracil.  RNA occurs primarily as a single polynucleotide strand, but it is fully capable of forming a double-strand structure, either with a complementary RNA strand or DNA strand.  Thymine in DNA is equivalent to uracil in RNA in forming complementary structures.  RNA has various functions in cells.  These include messenger RNA, transfer RNA, ribosomal RNA, and small nuclear RNA.  The nucleotide sequence of the various RNA varieties is coded in DNA, which serves as a template for RNA transcription.

ribose   A five-carbon sugar that occurs in the nucleotides of RNA.

ribosomal RNA   The RNA that occurs in ribosomes.  Ribosomes have a large amount of RNA that functions as such rather than coding for proteins.

ribosome   A small organelle found especially in association with the endoplasmic reticulum. Ribosomes are complexes of protein and RNA. They are the sites of all protein synthesis.

RNA   See ribonucleic acid.

RNA polymerase   The enzyme that catalyzes assembly of an RNA strand, using a DNA strand as the template. Strictly speaking, the full name should be DNA-dependent RNA polymerase, since some viruses can use an RNA template to make RNA.

schizophrenia   A complex of psychotic disorders characterized by disturbance of thought, mood, and behavior. Delusions and hallucinations are frequent.

secondary structure   In proteins, the structures that result when hydrogen bonds form between the hydrogen atom next to one peptide bond and the oxygen atom next to another. There are two stable structures that can form readily. One is a left-handed helix with hydrogen bonds formed between successive turns of the helix. The other is a "pleated sheet," in which the polypeptide chain folds back and forth in a plane to form parallel chains held together by hydrogen bonds between them. Both structures are common in native proteins.

segregation   A term that describes the separation of chromosomes and genes in meiosis.

segregation analysis   Analysis of the ratios of offspring from a particular parental cross to test for conformity with Mendelian theory. Either genotypes or phenotypes can be the object of segregation analysis.

selection coefficient   The loss of fitness of a particular genotype compared to the most fit genotype. This is expressed as s = 1 – w, where w is the fitness, with values from 0 to 1, and s is the selection coefficient. For the most fit genotype, w = 1 and s = 0.

semen   The suspension of spermatozoa in secretions of the seminal vesicles, forming the ejaculate.

semiconservative replication  A form of replication characteristic of DNA. When a double strand of DNA replicates, the two strands separate, and each serves as a template for the assembly of a new complementary strand. The two daughter DNA molecules produced each consist of one old strand and one new strand. The name semiconservative comes from the fact that one old strand is completely conserved and the other strand is completely new.

seminiferous tubules   Enclosed channels in the testes in which spermatozoa are produced. The spermatozoa move from the seminiferous tubules into the epididymis, where they are stored.

sense mutation   A nucleotide substitution in the termination codon that causes it to code for an amino acid rather than chain termination.  Translation of the mRNA continues beyond the usual stop point, and additional amino acids are added to the C-terminal end of the protein.

sense strand   One of the two strands of DNA in the coding region of a gene.  One strand serves as the template.  The other strand, which is complementary to the template strand, has the same nucleotide sequence as the primary transcript.  The messenger RNA is considered "sense."  Therefore, the DNA strand with the same information is also sense.  This strand is also called the coding strand.

Sertoli cells   Cells that line the seminiferous tubules and assist in the process of spermatogenesis. They also secrete Mullerian inhibiting hormone during embryonic development.

severe combined immunodeficiency (SCID)   Lack of both humoral and cellular immunity. Such an individual has no immune response and therefore no protection against either bacterial or viral infections.

sex   A means of genetic exchange, creating cells or individuals with new combinations of alleles. Sexual reproduction is in contrast to asexual reproduction, in which offspring are genetically identical to the parental cell or individual, there being only one parent rather than two.

sex chromatin   A darkly-staining body in the nuclei of cells from females.  This body, also called the Barr body after Murray Barr, one of the discoverers, is produced by inactivation and condensation of one of the two X chromosomes in females.  See X-chromosome inactivation.

sex chromosome   A chromosome that is involved in determination of sex. In mammals, females are produced when the chromosome complement includes two X chromosomes. Males are produced when there are one X chromosome and one Y chromosome. The X and Y chromosomes are therefore sex chromosomes.

sex-influenced trait   A trait whose expression is different in males and females.

sex-limited trait   A trait whose expression is limited to one sex, either male or female.

sex-linked   Any gene on the X chromosome. The preferred term is X-linked, in view of the presence of genes on the Y chromosome also.

sexual dimorphism   Phenotypic variation between sexes. In many species, there is relatively little difference in appearance of males and females. In others, the differences can be striking. In some species of birds, males have ornate plumage, whereas females are quite drab (by human standards).

sexual reproduction   Reproduction that is accompanied by exchange of genetic material. In mammals, the haploid gametes combine to form diploid offspring that are different from either diploid parent. Sexual reproduction is thought to be an advantage for a population because it generates new combinations of alleles, some of which are associated with superior adaptation. At the same time, some combinations would be less advantageous, and the alleles responsible would be eliminated.

siamang   One of the larger of the small apes, close relatives of gibbons.

sibship   A set of siblings (brothers and sisters).

sickle-cell anemia   An inherited disorder of hemoglobin structure, common in persons of African origin. Persons homozygous for the sickle-cell gene have red blood cells that form bizarre sickle shapes when oxygen is low, as in peripheral tissues. Such cells clog the circulation in blood capillaries, further reducing the oxygen and exacerbating the problem. The sickled cells also are more likely to rupture, releasing hemoglobin into the blood stream. Death usually occurs in the second decade. The genetic change is in the gene that codes for the ß-globin subunit of hemoglobin. The high early death rate among persons homozygous for the sickle-cell gene appears to be offset in some populations by the greater resistance of heterozygotes to falciparum malaria. See OMIM for technical details and links to other sites.

sickle-cell hemoglobin (Hb S)   The inherited variation in the beta-subunit of hemoglobin (Hb) that causes sickle-cell anemia in homozygotes.  The change is in the sixth amino acid from the N-terminus, which glutamic acid in the normal Hb A and valine in Hb S.  This causes the folding of the N-terminal region to change when the oxygen is low, as in peripheral tissues.  In high concentrations, the modified folding in turn causes aggregation of the Hb molecules into a crystal-like structure, which disrupts normal red cell morphology, forming so-called sickle cells.  Rupture of the red cells is a frequent result.

sickle-cell trait   The condition of heterozygosity for the sickle-cell gene.  Some one-third of the hemoglobin in the cells of heterozygotes is the variant Hb S.  In these concentrations, formation of sickle cells does not occur in vivo although it can be induced in the laboratory.  Phenotypically, persons with sickle-cell trait are normal.  The difference between having a "trait" and having "anemia" is not obvious to persons unversed in genetics and has caused much confusion and unnecessary concern.  Remember: sickle cell anemia = bad; sickle cell trait = not bad.

silent mutation   See synonymous mutation.

single nucleotide polymorphism (SNP)   Variations among chromosomes at a single nucleotide position, e.g. A on one chromosome, G at the same site on another. These may occur in coding regions of a gene, but most are in the noncoding regions, which comprise most of the chromosomes. SNP's in restriction sites would also be detectable as RFLP's. However, PCR primers can be designed that will bind to one SNP form but not the other. SNP's are inherited as codominant traits and are especially useful as genetic markers because of their very low mutation rate.

SNP   See single nucleotide polymorphism.

soma   [Greek soma body.] Soma is a general term meaning body. The adjective form is somatic. Common prefix and suffix forms are soma- and -some.

somatic cell   All cells of the body except germ cells.

somatic mutation   A mutation that occurs in somatic cells and that therefore cannot be transmitted to offspring.  Somatic mutations are important in genesis of cancer and certain other disorders.

Southern blot   A method, named after its inventor, E. M. Southern, for separating fragments of DNA and detecting the fragments that can bind a specific DNA probe. After gel electrophoresis, the gel is placed on wet filterpaper. The gel is covered by a nylon or nitrocellulose membrane, on top of which blotting materials (paper towels) are placed. The paper towels absorb liquid through the membrane, causing flow of liquid through the gel. DNA moves with the liquid but is stopped by the membrane, preserving the pattern of distribution of fragments originally in the gel. After the DNA on the membrane is denatured, the membrane is treated with a DNA probe to locate DNA bands that are complementary to the probe.

species   The most basic unit of biological classification. Members of a species share many characteristics that distinguish them from other species. In particular, members of the same species can interbreed to produce fertile offspring; members of closely related species can sometimes interbreed to produce offspring, which are typically sterile. [The word species is both singular and plural. The word specie refers to coins and not to a biological group.] See binomial nomenclature for additional levels of classification.

sperm   See spermatozoön.

spermatid   A male germ cell that has completed meiosis but that has not undergone the cellular conversions into a mature sperm (spermiogenesis).

spermatocyte   A male germ cell that has initiated spermatogenesis.

spermatogenesis   The process of forming haploid germ cells in males.

spermatogonia   Male germ cells that replicate by mitosis. Such cells are diploid and constitute the stem cell line in the testes that permits continual production of sperm throughout the adult life of a male.

spermatozoön   (Plural spermatozoa) A mature male germ cell. Commonly shortened to sperm.

spermiogenesis   The process of forming mature sperm from the haploid products of spermatogenesis.

spina bifida  A congenital condition in which the spinal cord is incompletely enclosed, resulting in extrusion from the spine.

spindle   The structure formed during nuclear division that participates in separation of chromosomes into daughter cells. The spindle consists of microtubules that radiate from two bodies (centrioles) that form the poles of the spindle. At metaphase, chromosomes are aligned in a plane between the poles and perpendicular to the spindle axis.

splicing   The process of removing introns from the primary transcript to produce messenger RNA.

spontaneous generation   The idea that living organisms can arise from nonliving matter. It is incorrect, at least for modern organisms. The earliest forms of life must have originated from nonliving systems, but that is quite a different story.

SRY   The gene on the Y chromosome of mammals that causes the gonads to develop into testes. The gene symbol (SRY) comes from the name sex-determing region Y. The presence or absence of the SRY gene is the primary determinant of sexual development, although other genes on the Y chromosome are also necessary for male fertility. Rare mutations at other loci, including autosomal loci, can cause sex reversal, but allelic variations at these loci do not contribute to normal sexual differentiation. Additional information on SRY is available in OMIM.

stem cell   A cell that replicates mitotically and that remains in a somewhat incompletely differentiated state. Stem cells serve as a source of cells that have the potential to differentiate but that have less or no potential to divide. Examples: bone marrow cells that can differentiate into precursors of red and white blood cells; spermatogonia in the testes that produce a continual supply of cells that differentiate into spermatozoa.

steroid   Any of a class of chemical compounds that have in common the steroid structure, a complex four-ring structure. Cholesterol is a steroid and is the starting point for biosynthesis of other important steroids, such as the adrenal hormones and the sex hormones (androgens, estrogens, and progesterone). Steroids are very hydrophobic and can pass readily through plasma membranes into cytoplasm. Their function as hormones is mediated by specific protein receptors in the cytoplasm, which form complexes that then enter the nucleus and regulate the activity of various genes.

stop codon   A codon that terminates translation rather than adding an amino acid.  Three of the 64 codons are stop codons; the remainder code for amino acids.

structural protein   A protein that acts to stabilize the architecture of a cell, its interior components, or multicellular arrays.  Examples include spectrin, on the interior of red blood cell plasma membranes, tubulin, in the microtubules of cytoplasm, and collagen and fibrillin, extracellular proteins that cause cells to adhere to each other.

substrate   The substance on which an enzyme acts. Often the product of one enzymatic reaction becomes the substrate for another.

sugar   Any of a group of chemical structures characterized by chains of three to seven carbon atoms, to most of which are attached one hydrogen atom and one hydroxyl group.  Because of the multiple hydroxyl groups, sugars are very hydrophilic and therefore soluble in water.  The technical term for sugars is saccharides.  Glucose is a common monosaccharide important in all life forms.  It is a hexose, meaning that there are six carbons in the chain.  Other hexoses are fructose and galactose, which differ from glucose in the three-dimensional arrangement of the H and OH groups.  Disaccharides are composed of two sugars connected by a covalent bond.  Examples are sucrose, which is a combination of glucose and fructose, and lactose, which is a combination of glucose and galactose.  Pentoses (five-carbon sugars) that are important players in biology are ribose, which occurs in the nucleotides of RNA, and deoxyribose, which occurs in the nucleotides of DNA.  Starch, glycogen, and cellulose are large polymers of glucose, the first two serving as energy stores in plants and animals, respectively.  Sugars, starches, and glycogen are carbohydrates.

synapsis   [Greek synapsis connection.] The pairing of homologous chromosomes during prophase I of meiosis. The pairing is highly specific, point-by-point matching. During this pairing, crossing over between nonsister chromatids occurs, generating new chromosomal combinations of alleles.

synonymous mutation   A mutation within a codon that consists of a nucleotide substitution that does not change the amino acid that is coded.  Same as silent mutation.

syntenic   [Greek syn with + tainia ribbon.] Descriptive of genetic markers that are on the same chromosome.

T-cell   See T-lymphocyte.

T-cell receptor   The receptor on the surface of T-lymphocytes responsible for binding to specific antigens.

T-lymphocyte   One of the two major types of lymphocytes, responsible for cellular immunity. T-cells are so-named because they mature in the thymus gland. There are two major types of T-cells: helper T-cells and cytotoxic (killer) T-cells. Cytotoxic T-cells attack cells that have foreign antigens on their surfaces and destroy (lyse) them. Helper T-cells are necessary in the action of B-cells in producing antibodies and cytotoxic T-cells in killing target cells. The interactions among these various types of lymphocytes is complex and depends both on direct contact and on chemical messages. T-cells also generate a small group of memory cells that create a rapid response in the event of a second exposure to the specific antigen.

tarsier   A small primate that occurs today only in parts of Southeast Asia. The tarsier is thought to be somewhat between other prosimians and monkeys in the development of higher primate characteristics. It is nocturnal, with large eyes facing forward, providing good stereoscopic vision in dim light.

Tay-Sachs disease   An autosomal recessive disorder involving deterioration of the central nervous system and death in young children. The gene that is responsible codes for the alpha subunit of the enzyme hexosaminidase A. This enzyme is in lysosomes and functions to breakdown certain complex molecules in the brain called gangliosides. The disease is rare except among Ashkenazic Jews, in whom the incidence is ca. 1 per 3600 births. Additional information can be found in OMIM.

telo-   [Greek telos end.] A common prefix to describe something at the end. Examples: telocentric, telophase.

telocentric   [Greek telos end + centric.] A chromosome in which the centromere is located at the end, producing a chromosome with only one arm. Humans have no telocentric chromosomes. Mice, however, have all telocentric chromosomes.

telophase   The final phase of nuclear division. The chromosomes uncoil and become very extended, a nuclear envelope forms around them, and the new nucleus enters interphase.

template   A form or mold used to create an object. In genetics, the term refers to a polynucleotide strand that is used to assemble a complementary strand. In DNA replication, both strands serve as templates for the assembly of complementary strands. In transcription, only one DNA strand serves as a template for the RNA copy that is made.

teratogen   [Greek teratos monster + genesis production.] Any agent that interferes with normal embryonic or fetal development. The term usually is applied to chemical or physical agents, such as alcohol or radiation. Variant genes can also be considered as teratogenic when they interfere with development.

tertiary structure   The final level of folding of polypeptide chains to form a functional, three-dimensional protein.

testicular feminization   See androgen insensitivity.

testis   A male gonad; the primary male sex organ. The testes, which are located in the scrotum, are the site of production of spermatozoa and the primary site of production of testosterone, the principal male hormone.

testosterone   The principal male hormone (androgen). Chemically, testosterone is a steroid. It is made in and secreted by the interstitial cells of the testes into the blood circulation. Testosterone and estrogens are also made in other tissues, especially the adrenal glands, in both males and females, but the levels are typically much lower than in the gonads.

tetramer   A structure that consists of four subunits.  Examples: tetrapeptide, tetranucleotide, or a protein complex composed of four proteins.

tetraploid   A cell or organism that has four sets of the basic haploid complement of chromosomes.  In the case of humans, there would be 92 chromosomes (4 × 23).

thalassemia   Any of a group of anemias due to inherited defects in the quantity of hemoglobin in the red blood cells.

thymine   A pyrimidine that occurs in the nucleotides of DNA.  Commonly abbreviated T in DNA sequences.

totipotent  The state of having full potential for development. The term is used to describe early embryonic cells that have not differentiated and thus have the potential to differentiate into any tissue.

Tourette syndrome   Also known as Gilles de la Tourette syndrome, this disorder involves facial and vocal tics, jerky movements, echolalia (involuntary repeating of words said by others), and coprolalia (involuntary use of obscenities). See OMIM.

transcription   Formation of an RNA copy of a DNA strand. Transcription is catalyzed by RNA polymerase, in this case DNA-dependent RNA polymerase.

transcription factor   Any factor, usually protein or a protein complex, whose presence alters the rate of transcription, either up or down.  The factor may bind directly to DNA, usually in the 5'-flanking region but sometimes more distant, or to other transcription factors that bind DNA.  Most transcription factors are quite specific for their target genes.

transfer RNA (tRNA)   A small RNA whose function is to recognize the genetic code and add the proper amino acid to the nascent polypeptide chain during translation.  Each amino acid will bind to one or a few tRNAs.  Each tRNA will bind only to one amino acid.  When the codon for a particular tRNA shows up on the docking site of the ribosome, the anticodon of that tRNA binds to the mRNA.  At the other end of the tRNA, the amino acid that is attached breaks the bond to the tRNA and forms a peptide bond with the previous amino acid added.  The tRNA is then released and recycles.

transformation   A general term meaning to change from one form to another.  It is often used in genetics to refer to a change in phenotype that results from, or possibly results from, a change in genotype. In the context of cancer, transformation refers to the change from regulated growth (normal) to unregulated growth (cancer).

transgenic   An organism into which DNA from a different species has been incorporated into the genome.

translation   The process of transferring the information coded in messenger RNA into the corresponding amino acid sequences of a polypeptide.  Translation occurs on ribosomes in the cytoplasm.  The process requires messenger RNA, aminoacyl transfer RNAs, in addition to the very complex machinery of the ribosomes.

translocation   Breaks in each of two chromosomes or chromatids that exchange distal segments before repair.  The result is no loss of chromosomal material, but genes that formerly were on one chromosome are exchanged with genes on another chromosome.  The karyotype formula includes t(m;n) to indicate translocation between chromosomes m and n.  E.g. 46,XY,t(13;15) indicates that a translocation has occurred between chromosomes 13 and 15.

transport protein   A protein whose function is to bind and transport a substance from one part of the body to another.  Examples are hemoglobin, which transports oxygen from the lungs to the tissues, and transferrin, which binds iron in the plasma and transports it to tissues, especially liver.

tree shrew   Small primates (prosimians) that live in trees in southeast Asia. Their diet is primarily insects. Although superficially they rather resemble squirrels, close anatomical analysis shows their clear relationship to primates. The ancestral stock of the primates is thought to have been much like the present-day tree shrews.

trinucleotide expansion   An event that occurs at rare loci that have repetitive sequences of three nucleotides, such as –(CAG)n–.  If n exceeds a threshold value, which is different for each locus, it increases during transmission to offspring, sometimes to very large values.  The mechanism is not fully understood, and the increase can be in males only, females only, or both, depending on the locus.  Below the threshold value, n is stable and does not increase during transmission.  Typically, there are values of n above the threshold that have no phenotypic effect but are subject to expansion upon transmission.  These ranges of n are described as premutations, whereas values of n that affect the phenotype are called full mutations.  The greater the value of n within the premutation range, the more likely transmission is to cause a full mutation.  This generates pedigrees in which anticipation is observed, i.e. the mutant phenotype appears at younger ages in succeeding generations and often with greater severity.  The fragile-X syndrome and Huntington disease are examples of genetic disorders that commonly result from trinucleotide expansions, though more conventional mutations are also known in the case of fragile-X syndrome.

triploid   A cell or organism that has three sets of the basic haploid complement of chromosomes.  In the case of humans, there would be 69 chromosomes (3 × 23).

triplo-X syndrome   The presence in females of an extra X chromosome.  The karyotype formula is 47,XXX.  The extra X chromosome results from nondisjunction, which could occur either in the mother or the father, though it appears to be most often maternal.  Imbalance of function of genes on the X chromosome is largely prevented by X-chromosome inactivation, and triplo-X females are essentially normal and fertile.  Contrary to expectation, there is no excess of 47,XXX or 47,XXY syndromes among their offspring.

trisomy   Having three copies of a particular chromosome, i.e. the total number of chromosomes in a trisomic cell is 47.  Trisomy arises by means of nondisjunction.  Most human autosomal trisomies are aborted spontaneously.  However, trisomies 13, 18, and 21 (Down syndrome) do occur among newborns.  Trisomy should not be confused with triploidy, which involves three complete sets of chromosomes, a total of 69 chromosomes.

trisomy 21   See Down syndrome.

trophoblast   The spherical shell of cells that comprise the blastocyst with the exception of the few that are destined to form the inner cell mass that develops into the embryo. The blastocyst cells invade the walls of the uterus and participate in formation of the placenta. They are genetically identical to the embryo, both being products of the same zygote.

tumor   An abnormal growth of tissue. Tumors may either be benign or malignant. Malignant tumors are known as cancer.

tumor progression   The change in the characteristics of cancer from a slow-growing, therapy-sensitive form to a rapidly-growing, invasive, therapy-resistant form. Progression is thought to be primarily due to accumulation of mutations, each of which makes the tumor more aggressive.

tumor suppressor gene   A gene that greatly increases risk of cancer when a cell becomes homozygous for nonfunctioning alleles. The presence of at least one functional allele therefore "suppresses" cancer, provided there are not other genetic changes in the cell that "cause" cancer. The retinoblastoma locus is a classic example of a tumor suppressor gene.

Turner syndrome   Also known as gonadal dysgenesis, Turner syndrome is due to a chromosome complement of one X chromosome plus the normal autosomal complement.  The karyotype formula is 45,X (sometimes written 45,X0).  Affected persons are females, with short stature, typical facial appearance and webbed neck, and fibrous streaks in place of ovaries.  They have normal intelligence, although there is a specific deficit in the spatial memory.

unbalanced translocation   A chromosome complement that contains a translocation with hemizygosity for some chromosome segments.

unequal crossover   A crossover between mispaired nonsister chromatids.  The result is duplication in one chromatid of the segment between the crossover points and deletion of that segment in the other chromatid.  Mispairing appears more likely to occur if there is some similarity of DNA sequence between the mispaired segments.

uniparental disomy   The condition in which the two members of a pair of homologous chromosomes both come from the same parent.  There are several possible mechanisms for the origin of uniparental disomy: (1) joining of two gametes, one with deficiency of a particular chromosome, the other with two copies of that chromosome; (2) trisomy of a  chromosome in the zygote, with early loss of one chromosome to restore the euploid state; (3) monosomy for a chromosome with duplication of that single chromosome to restore the diploid state.  See isodisomy and heterodisomy.

universal code   Refers to the fact that all forms of life use the same genetic code.

uracil   A pyrimidine that occurs in the nucleotides of RNA.  Commonly abbreviated U in RNA sequences.

vaccination   The introduction of a bacterial or viral antigen into a person or other animal in order to stimulate immunity against those antigens. Typically, the antigen is either a preparation or extract of the killed bacteria or viruses, or it is an infection with live bacteria or viruses that have been modified so as not to cause disease.

valence   The characteristic number of chemical bonds that an atom can form.

variable number of tandem repeats (VNTR)   Repeated nucleotide sequences that differ among homologous chromosomes in the number of repeats. See minisatellite, microsatellite.

variable region   The N-terminal regions of thee H and L chains of antibody molecules. This region is the part of the antibody that reacts with a specific antigen. The great variability reflects the antibody diversity that is necessary to combine with very diverse antigens.

vas deferens   See ductus deferens.

viral oncogene   See oncogene.

virus   A particle that contains some of the genes required for self replication but that cannot function outside a cell. Some viruses use DNA as the genetic material; others use RNA. Some destroy the cell that they infect; others integrate into the DNA of the cell and replicate with the host DNA. Viruses are generally not considered to be living, since they cannot function outside a cell. However, some do seize control of cell machinery, subverting it to the virus's own purposes.

Wallace   Alfred Russel Wallace (1823-1913), independently of Darwin, arrived at the idea of natural selection in 1858. Upon communicating these ideas to Darwin, both Wallace's and Darwin's were made public in 1858, and both are given credit for them, although Darwin's subsequent publications and presence in England rather than in the Southwest Pacific made him the more visible figure to the public.

Watson-Crick model of DNA   Although DNA was discovered in the 1870's by Friedrich Miescher, and the structure of polynucleotide chains was established in the first half of the Twentieth Century, this in itself was not sufficient to understand how DNA stored genetic information, replicated it, and transferred it to the cytoplasm to provide instructions for cell processes.  In 1953, James D. Watson and Francis Crick published two very short papers that proposed an additional level of structure that proposed solutions to these problems.  Their proposals opened the age of molecular biology, an age that has yielded rich benefits and that is expanding still.  Their model of DNA consisted of two polynucleotide chains oriented in opposite direction and held together by hydrogen bonds.  The model can be visualized as a ladder, with the sugar-phosphate chains constituting the outside supports and the bases pairing between the supports and forming the rungs.  Hydrogen bonds are weak and can form only if the orientation of the participants is just right.  In the case of DNA, "just right" occurs when adenine and thymine pair and when guanine and cytosine pair.  Thus and A on one ladder support must be paired with a T on the other to form a rung, and G must be paired with C.  A sequence such as -G-T-T-C-A-C- on one half of the ladder would pair with -C-A-A-G-T-G- on the other.  These sequences are described as complementary, much in the way that a film negative and a print made from it are complementary, a dark spot on the film generating a light spot on the print and vice versa.  Because of the three-dimensional structure of the DNA components, the ladder twists, generating a double helix (a helix with two strands) that is right handed, i.e. when viewed down the axis of the helix, the twist is clockwise as it moves away from you.  The two polynucleotide strands have "direction" due to asymmetry of the nucleotides.  The direction is indicated by labeling one end 5' and the other 3'.  (These numbers come from conventions that chemists have in labeling atoms in a molecule.  At one end, there is a hydroxyl group on the 5' carbon of the deoxyribose, and at the other end, there is a hydroxyl group on the 3' carbon of the deoxyribose.  It will be sufficient for the moment if you just remember that one end of a polynucleotide chain is 5' and the other 3'.)  The double helix will form only if the two strands are antiparallel, i.e. if one is 5' to 3', the other must be 3' to 5'.  Watson and Crick proposed that information is stored in DNA by the sequence of nucleotides, much as computers store information as a string of 1's and 0's.  They further proposed that DNA replicates semiconservatively, solving one of the large requirements of DNA structure.

wild-type   A term used to describe the common, usually normal alleles of a gene.

Wolffian duct   Embryonic ducts that, under the influence of androgens, develop into internal male structures (epididymis, ductus deferens, seminal vesicles). In females and in persons with androgen insensitivity, the Wolffian ducts degenerate.

X-chromosome inactivation   Inhibition of the activity of genes on all but one of the X chromosomes in individuals with two or more X chromosomes. In 46,XX females, only one of the two X chromosomes is fully active, the other being largely inactive. Thus males with one X chromosome and females with two have the same effective gene dosage for genes on the X chromosome. Inactivation occurs early in embryonic development, the choice of which X chromosome to inactivate being random in each cell. Once inactivated, a particular X chromosome remains inactive throughout subsequent mitotic cell divisions.

X-linked   Refers to any gene on the X chromosome. Equivalent to sex-linked.

XX male   A male with two X chromosomes and no apparent Y chromosome. Most cases are due to translocation of the SRY gene on the Y chromosome to an X chromosome.

XY female    A female with an X and a Y chromosome rather than two X chromosomes. Among the common causes are mutations in the male-determining SRY gene and mutations in the androgen receptor gene.

XYY syndrome   A condition in which males have an extra Y chromosome, with karyotype formula 47,XYY.  The extra Y chromosome arises from nondisjunction in the father.  Affected males are slightly taller than other males of the same genetic background, and some appear to be more impulsively aggressive and violent than normal.  They are of normal intelligence, and most do not have major behavioral problems.  Approximately one per thousand males is 47,XYY.

xenotransplant   Transplantation of an organ or tissue to a member of a different species.

xeroderma pigmentosum   A group of recessive traits that result from defects in repair of certain DNA alterations.  Affected persons are unable to remove thymine and cytosine dimers, which form on exposure to ultraviolet light.  This greatly increases the mutations in skin exposed to sunlight and leads to multiple skin cancers.

Y-linked Refers to any gene on the Y chromosome.

yeast artificial chromosome (YAC)   A "chromosome" that has been assembled by joining together DNA fragments that contain the elements of a chromosome, such as a centromere, telomeres, and replication origins, along with other DNA of interest. The YAC is then introduced into yeast cells, where it replicates as part of the normal replication of chromosomes. The advantage of YAC's is the very large pieces of DNA that can be inserted, much larger than in the case of bacterial vectors.

zinc finger   One of several motifs (DNA or protein sequences associated with certain functions) that occur in proteins that bind directly to DNA.  The name is derived from the way the polypeptide chains fold, producing multiple loops joined at their bases by an atom of zinc.  When this motif is encountered in a DNA sequence, the presumption is that the protein product functions as a transcription factor.

 zygote [Greek zygotos joined together.] A fertilized egg.

Back To: 
ZOO 317 / Course Info / Lecture Sched / On-line Resources

last revision: 1 October 1998
owned by: Dr. Eldon Sutton