II. X-linked traits.

III. Y-linked traits.

IV. Pseudoautosomal inheritance.

Terms

Notes (1) The term sex linkage has been used historically as equivalent to X-linkage, there having been no known functional genes on the Y chromosome. While X-linkage is the preferred term now, sex-linkage is still encountered in the meaning of X-linkage. (2) Page 89, definition of hemizygous: This word is defined more generally than suggested here. It refers to any chromosome or chromosomal segment for which there is no homolog present. (3) Page 107, Science and Society item 2: The pedigree of ichthyosis described has been proved to have been modified for purposes of show business. The correct version is inconsistent with Y linkage.

I. Transmission of autosomal dominant traits.

A. Autosomal dominant inheritance refers to those situations in which a single copy of an allele is sufficient to cause espression of a trait.
1. Every affected person should have at least one affected parent.

2. Males and females should be equally often affected.

3. An affected person has a 50% chance of transmitting the dominant allele to each offspring.

4. For very rare alleles, it may be that persons homozygous for the dominant allele have never been observed. The homozygous dominant phenotype may be different from the heterozygous phenotype.

B. Examples of autosomal dominant traits include:
1. Brachydactyly, which was the first example in humans of a trait transmitted according to Mendelian rules. Persons with brachydactyly have very short fingers.

2. Marfan syndrome, a dominantly inherited defect in connective tissue. Early death often results from rupture of heart vessels.

3. Achondroplasia, a form of dwarfism with normal trunk size but short arms and legs.

4. ABO blood groups, with alleles I^A and I^B dominant to I^O and codominant to each other.

5. Familial hypercholesterolemia, a defect in regulation of blood cholesterol that causes high levels of cholesterol in heterozygotes and very high levels in homozygotes.

6. Huntington disease, involving a type of late onset deterioration of the central nervous system.

C. An exception to I.A.1 may occur because of mutation.
1. Mutation is any sudden appearance of a heritable allele that was not originally present in the biological parents.

2. Mutation is very rare, on the order of 1/100,000 to 1/1,000,000 per locus per gamete. Therefore, if a person has a dominant trait that is not present in the parents, mutation should be invoked as an explanation only if all other reasonable explanations have been ruled out.

A. Females transmit an X chromosome both to sons and daughters; however, males transmit their X chromosomes only to daughters and their Y chromosomes only to sons.

B. For rare X-linked dominant traits:

1. Twice as many females are affected as males.

2. Half the children of an affected female will be affected, regardless of sex.

3. All the daughters of an affected male will be affected but none of the sons.

C. For rare X-linked recessive traits:
1. Hemizygous males are affected, but only homozygous females are affected. The ratio of affected males-to-females is >>1.

2. Affected males will transmit the gene to all daughters, who will usually be heterozygous and therefore not affected, but to no sons.

3. Affected males in a pedigree are related to each other through nonaffected females; e.g. grandfather—mother—son or maternal uncle—mother—son.

4. Homozygous recessive females can arise only from matings in which the father is affected and the mother is heterozygous or homozygous.

D. Examples of X-linked recessive traits are:
1. Colorblindness.

2. Hemophilia.

3. Duchenne muscular dystrophy.

A. Since males are hemizygous for Y-linked genes on the nonhomologous part of the Y chromosome, all will be expressed.

B. Y-linked genes are transmitted from father to son, never to daughters.

C. There are no essential genes on the nonhomologous portion of the Y; however, the gene for maleness is there, as is a locus concerned with male fertility.

A. The tips of the short arms of the X and Y chromosomes are homologous and pair during meiosis, with one crossover. Genes in the homologous segment show pseudoautosomal inheritance.

B. Genes known to be pseudoautosomal are very rare, but this may be due to difficulty in distinguishing them from autosomal genes.

C. There is also a short region of homology at the tips of the long arms of the X and Y chromosomes, but no genes are known there, and crossing over appears not to occur.

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