Advances in molecular biology have given rise to a new era in medical diagnostics. Physicians will soon be able to detect many serious diseases, or determine high genetic risk for them, years before symptom onset. Prenatal and presymptomatic genetic tests already exist for several single-gene illnesses, including Duchenne muscular dystrophy, cystic fibrosis, polycystic kidney disease, and Huntington's disease. Tests indicating susceptibility to certain forms of cancer, heart disease, Alzheimer's disease, bipolar affective disorder, alcoholism, and other common illnesses are likely to be developed in the not-too-distant future.

Although predictive DNA tests have the potential to revolutionize the practice of medicine, their social, economic, and ethical complexities are only now beginning to be seriously addressed (Holtzman, 1989). This chapter will outline some of the ethical issues involved in predictive genetic testing, and illustrate them with examples from a program of testing for Huntington's disease at the Johns Hopkins University School of Medicine. One of the primary objectives of this program is to provide sound empirical data to serve as a basis for ethical decision making in the area of genetic diagnosis.

The decision to be tested

Under virtually all circumstances, informed consent should be obligatory in genetic testing of adults for future disease vulnerability. The reasons for this are several. First, predictive genetic tests represent a new and innovative, if not experimental, technology. They are not yet part of routine clinical practice, and our experience with them is limited. Secondly, such tests are inherently elective and discretionary.