Zoo 317 Heredity, Evolution and Society
|Lecture 34||No text|
II. Local polymorphisms.
III. Local disorders.
I. Many loci are polymorphic world-wide. Ca. 50% of the genes that code for enzymes are polymorphic, with somewhat lower frequencies for genes whose products have other functions. Among the best studied of the world-wide polymorphic genes are the following:
2. O is the most common allele in most populations. A occurs in most populations and is quite high in some, the major exception being Central and South American Indians, in whom it is absent. (But there is a center of high A frequency in Western Canada.) The B allele is highest in Asia but is absent in the closely-related Amerinds.
2. The Ro allele (one of several alleles that comprise the Rh-positive "allele") is found primarily among persons of African ancestry. Other major Rh+ alleles are found in all populations, although the frequencies vary.
D. Lactase persistence is a dominant trait that varies from very high frequencies in Scandinavian countries to very low frequencies in Asia. Lactose is the sugar in milk. Lactase is an enzyme required to break it down. In most mammals, production of lactase in the intestinal tract ceases after the usual period of nursing. This is also true of most humans. After that time, the lactose in fresh milk cannot be broken down properly, and the bacteria in the gut have a great time with it, causing great discomfort to the host. If a person is either homozygous or heterozygous for persistent lactase, there is no problem. There are now various processed milks and things to add to whole milk to remove lactose for the benefit of persons who lack lactase.
B. The Dia allele of the Diego blood group system is found only in Asians and Amerinds and supports the close genetic affinity of these populations.
C. Sickle-cell anemia occurs primarily among persons of African ancestry. It also occurs in a few groups in the Mediterranean area, as well as in small groups in the Arabian Peninsula and India. The Hb S allele occurs at very low frequencies if at all in other populations. Other Hb mutations, such as Hb E and thalassemia mutations do occur in polymorphic frequencies in some of these other populations.
D. G6PD deficiency (glucose 6-phosphate dehydrogenase, an enzyme that helps metabolize the sugar glucose) occurs primarily in persons of African ancestry. It is an X-linked recessive trait, with occasional expression in heterozygous females. The deficiency has no effect on the phenotype unless the person is exposed to certain drugs, in which case it can cause destruction of red blood cells. Other alleles of the G6PD locus are polymorphic in some other populations. Of particular note is an allele that occurs primarily in Italy and the Eastern Mediterranean that causes males to be very sensitive to fava beans.
B. Cystic fibrosis is rare except among persons of European ancestry.
C. Congenital dislocation of the hip occurs with very high frequency among the Navaho. The cause is unknown, but the disorder is probably a complex inherited trait.
D. Diabetes mellitus type 2 occurs with high frequency among certain Indian groups, e.g. the Alabama-Coushatta in East Texas and especially the Pima Indians of Arizona.
E. U.S. Blacks have a high frequency of hypertension and heart disease of unknown cause. It is thought by most to be inherited.
F. The high risk of breast cancer in some families is associated with inheritance of mutations at either the BRCA1 or BRCA2 locus. Many different mutations have been found at these loci. Most are individually rare. However, several occur at high levels in certain populations.
2. The BRCA2 mutation 6174delT is found in 8% of Ashkenazic Jewish women who have breast cancer.
3. BRCA2 mutations also carry a risk of breast cancer in males. In Iceland, 40% of males with breast cancer inherited a 5 bp deletion in BRCA2. This particular mutation, but not others, occurs with high frequency in Iceland.
|ABO blood groups||Rh blood groups||MNSs blood groups||lactase persistence|
|lactose||Duffy blood groups||Diego blood groups||sickle-cell anemia|
|G6PD deficiency||Tay-Sachs disease||cystic fibrosis||hypertension|
|congenital dislocation of the hip||diabetes mellitus type 2|