CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

12. Diversity Within Species and Population Genetics

12.7. Human Population Genetics

Recall from earlier in this chapter that the human gene pool consists of a number of subgroups. The particular characteristics that set one group apart from another originated many thousands of years ago, before travel was as common as it is today, and we still associate certain physical features with certain geographic areas. Although there is much more movement of people and a mixing of racial and ethnic types today, people still tend to have children with others who are of the same social, racial, and economic background and who live in the same locality.

This non-random mate selection can sometimes bring together two individuals who have alleles that are relatively rare. Information about allele frequencies within specific human subpopulations can be very important to people who wish to know the probability of having children with certain harmful genetic abnormalities. This is important if both individuals are descended from a common ancestral tribal, ethnic, or religious group. For example, Tay-Sachs disease causes degeneration of the nervous system and the early death of children. Because it is caused by a recessive allele, both parents must pass on the allele to their child in order for the child to have the disease. By knowing the frequency of the allele in the backgrounds of both parents, the probability of their having a child with this disease can be determined.

Ashkenazi Jews have a higher frequency of the recessive allele for Tay-Sachs disease than do people of any other group of racial or social origin, and the Jewish population of New York City has a slightly higher frequency of this allele than does the worldwide population of Ashkenazi Jews (figure 12.15). Therefore, people with this background should be aware of the probability that they will have children who will develop Tay-Sachs disease, even though the allele is moving to populations other than the Ashkenazi.

FIGURE 12.15. The Frequency of the Tay-Sachs Allele

The frequency of an allele can vary from one population to another. Genetic counselors use this information to advise people of their chances of having specific alleles and of passing them on to their children.

Likewise, sickle-cell anemia is more common in people of specific African ancestry than in any other human subgroup. Because many black slaves were brought to this country from regions where sickle-cell anemia is common, African Americans should be aware that they might carry the allele for this type of defective hemoglobin. If they carry the allele, they should consider their chances of having children with this disease. These and other cases make it very important that trained genetic counselors have information about allele frequencies in specific human ethnic groups, so that they can help couples with genetics questions.

HOW SCIENCE WORKS 12.1

The Legal Implications of Defining a Species

The red wolf (Canis rufus) is an endangered species, so the U.S. Fish and Wildlife Service has instituted a captive breeding program to preserve the animal and reintroduce it to a suitable habitat in the southeastern United States, where it was common into the 1800s. Biologists have long known that red wolves hybridize with both the coyote (Canis latrans) and the gray wolf (Canis lupus), and many suspect that the red wolf is actually a hybrid between the gray wolf and coyote. Gray wolf- coyote hybrids are common in nature where one of the species is rare. Some have argued that the red wolf does not meet the definition of a species and should not be protected under the Endangered Species Act.

Museums have helped shed light on this situation by providing skulls of all three kinds of animals preserved in the early 1900s. It is known that, during the early 1900s, as the number of red wolves in the southeastern United States declined, they readily interbred with coyotes, which were very common (the gray wolf had been exterminated by the early 1900s). Some scientists believe that the skulls of the few remaining "red wolves" might not represent the true red wolf but a "red wolf" with many coyote characteristics. Studies of the structure of the skulls of red wolves, coyotes, and gray wolves show that the red wolves were recognizably different and intermediate in structure between coyotes and gray wolves. This supports the hypothesis that the red wolf is a distinct species.

DNA studies were performed using material from preserved red wolf pelts. The red wolf DNA was compared with coyote and gray wolf DNA. These studies showed that red wolves contain DNA sequences of both gray wolves and coyotes but do not appear to have distinct base sequences found only in the red wolf. This evidence supports the opinion that the red wolf is not a species but may be a population that resulted from hybridization between gray wolves and coyotes.

There is still no consensus on the status of the red wolf. Independent researchers disagree with one another and with Fish and Wildlife Service scientists, who have been responsible for developing and administering a captive breeding program and planning the reintroductions of the red wolf.

12.7. CONCEPT REVIEW

18. Give an example of a human population with a high frequency of a deleterious allele.

19. What is the role of a genetic counselor?