It is increasingly fashionable in medical circles, as this first-rate summary reminds us, to view disease as the result of the interaction between genetic predisposition and environmental factors. At one extreme are conditions like Huntington's disease where a single defective gene inexorably gives rise to symptoms later in life. Elsewhere along the interactive spectrum are diseases like juvenile diabetes, multiple sclerosis, cancer, and schizophrenia that are considered multifactorial: some combination of genes plus environmental variables--a virus attack, exposure to chemicals, stress, etc.--trigger the onset of the disease. Geneticist Harsanyi (Cornell Medical Center, New York) and science-writer Hutton clearly and authoritatively explain just where we stand in predicting who might get what disorder, given which conditions. Often the prediction is based on the presence of genetic markers--genes that can be implicated in the disease mechanism, but often are either contiguous with the disease-causing gene (on the same chromosome) or in some other way inherited along with the predisposing factor. Among those markers are the HLA antigens--a set of proteins that dot the surface of body cells and serve to distinguish your own cells from anyone else's. (These antigens, as well as blood types, are used in matching donors and recipients for organ transplants, for example.) The authors list the many disorders now associated with particular markers, discussing certain ones in great detail. The links are not causal, to be sure, and in some cases they are tenuous. (Wet earwax, for example, is associated with higher rates of breast-fluid secretion; breast fluid can concentrate carcinogens, etc.; and so Caucasian women, who tend to have wet earwax, have higher rates of breast cancer than do Asian women, with dry earwax.) Be that as it may, the authors are manifestly pointing to a wave of the future that raises disturbing social, legal, and moral issues. Do you abort a fetus destined to develop a disease in mid-life? Do you refuse employment to someone likely to develop skin allergies or lung cancer when exposed to certain chemicals? For their part, the authors take pains to explain the neutrality of the gene--the ""bad"" sickle cell trait is ""good"" if you live in a malarial zone. But their professional interests clearly emphasize the determining power of heredity. An important book--both for its currency and for its implications.