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Assessing Genetic Risks: Implications for Health and Social Policy
The most common autosomal recessive disorders include CF (1 in 25 carriers in the Caucasian population), sickle cell anemia (1 in 12 carriers in the African-American population), thalassemia (variable high carrier rates in Asian, Mediterranean, and Middle Eastern populations), and Tay-Sachs disease (1 in 30 carriers in Jews of Ashkenazic descent).
Genetic drift, or variation in the frequency of genetic traits due to chance, is one of the reasons for a higher than average frequency of specific recessive diseases in a specific ethnic group. Another reason may be some unknown protective benefit of the carrier state (e.g., sickle cell trait conferring resistance to malaria), or the origin of the trait in a common ancestor (founder effect), or a combination of both mechanisms, a well-known example of which is the high frequency of Tay-Sachs disease in Ashkenazi Jewish populations. As a result of the higher population frequency of these disorders, much effort has been expended in search of the genes for these diseases. In some cases, the altered gene has been identified; in other cases, biochemical assays serve as indirect indicators of the carrier or disease state.
Carrier testing can also be conducted for X-linked recessive disorders, such as hemophilia, Duchenne muscular dystrophy, or fragile X syndrome (see Box 2-1 for discussion of fragile X syndrome). In these cases, the mother of an affected son—who has no other affected relatives—might desire carrier testing to confirm whether the disease is a heritable trait or a "fresh" or new mutation (more common in X-linked recessive disorders than in autosomal recessive disorders); the sister, aunt, or female cousin of an affected individual might also seek carrier testing.
Heterozygote detection is intended to identify carriers of one copy of the mutant gene. Persons who carry a single copy of the gene for a recessive disorder do not have that disease, nor are they symptomatic. Carrier testing involves individuals known to be at high risk of being carriers because of family history; it is almost always conducted in a specialized medical setting. Carrier screening involves individuals with no previous family history in order to determine their risk; such screening programs are usually not conducted in a specialized medical setting. Testing and screening to detect carrier status for autosomal recessive disorders are used primarily to aid in informed reproductive planning and decision making.
When at-risk couples are both identified as carriers for autosomal recessive disorders, they have several options available to them:
they can take their chances, understanding there is a 75 percent likelihood with each pregnancy that the child would not be affected with the disease, or a 25 percent likelihood that each child will be affected;
they can avoid reproduction, consider adoption, and be fully informed about contraception or sterilization;
they can conceive through gamete donation, which ideally includes screening of a potential sperm or ovum donor to rule out carrier status for the same trait;