• they can undergo prenatal diagnostic testing using amniocentesis or chorionic villus sampling (CVS) and selectively abort an affected fetus;

  • they can undergo prenatal diagnostic testing using amniocentesis or chorionic villus sampling and make preparations for the birth of an affected child; or

  • although still experimental, it may soon be possible to undergo in vitro studies in gametes (ova and polar bodies) or in early blastomeres to permit selective uterine implantation of embryos that are not homozygous for currently diagnosable genetic disorders.

Sickle cell anemia, CF, and Tay-Sachs disease are examples of recessive disorders for which carrier testing and screening are available. Carrier detection for each of these disorders raises unique issues (described in Chapter 1). Sickle cell and Tay-Sachs screening programs of the past have illuminated the following issues: (1) the need to recognize social and cultural differences in screening highrisk populations; (2) the need for reliable, easy, and relatively inexpensive carrier tests; (3) the importance of people understanding the benefits and risks from participating in screening; and (4) the importance of pre- and posttest education and counseling. The setting of carrier screening programs may also be a critical factor in their success because it may affect community trust and confidence in such screening programs.

Population carrier screening for cystic fibrosis would be unprecedented in the United States in terms of the number of individuals who could potentially be tested. CF is the most common, potentially lethal autosomal recessive genetic disorder in the United States. Current population data indicate that 1 in 2,500 newborns of European ancestry is affected by CF (about 30,000 persons), and that 1 in 25 (about 15 million Americans) is an unaffected carrier of a single copy of the gene for CF (Collins, 1992; OTA, 1992). CF causes chronic lung disease and pancreatic insufficiency, characterized by excessive production of thick mucus, primarily in the lungs, and increased risk of infection, although there is a wide range in severity of symptoms. Men with CF are usually sterile; women with CF have low fertility rates and pregnancy may exacerbate their disease. In the past, many children with CF died in early childhood. There is currently no cure available, but with improved treatments, median survival for persons born with CF in 1964 has increased to age 29 (OTA, 1992). Some individuals with CF are living into their forties and fifties. Intensive research efforts are under way to develop a variety of drug and gene therapy approaches, including gene replacement, to the treatment of CF, but the success of these efforts is still being studied at this time (ASHG, 1992).

The most common mutation causing CF is known as delta F508, a deletion of an amino acid at the 508 codon in the gene (Collins, 1992). It accounts for about 70 percent of CF mutations among those of European ancestry. Six to twelve other mutations account for an additional 15 to 20 percent of all mutations. More than 230 mutations have already been identified (ASHG, 1992), and experts sug-



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