of genetic testing. One such concern is that those who are poor or live in rural areas will not have access to testing. The development and widespread use of genetic tests before safe and effective treatments are available have raised fears about the uses of genetic testing—that genetic tests will be imposed while other approaches to alleviating human suffering are neglected. Their use also raises issues about discrimination and privacy—that people found to possess certain genetic characteristics will lose opportunities for employment, insurance, and education. Finally, genetic testing raises worries about inequities and intolerance—that not everyone will share equitably in the benefits of genetic testing, that some will be stigmatized, and that the beauty of human diversity will be denigrated due to a narrowed definition of what is acceptable.

This is not the first time the National Academy of Sciences (NAS) has addressed issues related to genetic testing. Since Genetic Screening: Programs, Principles and Research was published by the academy in 1975 (NAS, 1975), the technology underlying genetic testing has been revolutionized, greatly expanding the number of diseases for which genetic testing will be possible. This has brought new urgency to many of the issues raised in the 1975 report and raised some additional issues as well. In this chapter, we briefly review the technological changes and their implications for assessing genetic risks. We revisit the first NAS report, and consider its applicability today.


During the 1970s, researchers discovered that human DNA (as well as DNA from other organisms) could be cut reproducibly into small segments, each of which could then be rapidly reproduced (cloned) by inserting it into a microorganism. Cloned segments can then be prepared in sufficient quantity to serve as "probes" to find the longer piece of human DNA from which the segment has been cut. Further advances facilitate determination of the chromosome on which the segment of DNA resides.

The availability of DNA probes made it possible to search for individual variation in DNA. Different kinds of DNA variation have been found. Probes can detect such DNA sequence variation among individuals (polymorphisms), and the inheritance of chromosome segments containing polymorphisms can be easily traced from parents to offspring (Botstein et al., 1980). Using DNA sequence variations as "markers," researchers began constructing maps defining their order and spacing along chromosomes (e.g., Donis-Keller et al., 1987). An important application of this genetic mapping technology was the localization at specific chromosomal sites of genes responsible for inherited disorders.

If, for instance, within a large family the relatives who had a genetic disease were found to have one form of a polymorphism significantly more often than blood relatives who did not have the disease, the disease-causing gene (still uni-

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