An accessible catalogue of some twenty hereditary disorders and an entreaty for genetic education. Greenblatt, a medical writer, points out that there are about 2000 known inherited abnormalities, some fatal, others treatable. Using the ""case history"" and ""medical detective"" approaches to exposition, she discusses Tay-Sachs and sickle cell diseases, Rh incompatibility, hemophilia, cystic fibrosis, and others, pointing out ethnic and racial factors. Somewhat peremptorily, Greenblatt includes schizophrenia and depression among rite lot. But she carefully debunks the Schockley-Jensen contention that I.Q. is innate and static: environmental factors are important, and cultural bias infuses the tests. Moreover, she takes on the myth that the extra Y chromosome in men produces violent behavior; it may bring on impulsiveness, she argues, but this is not tantamount to criminality. People do not understand genetic pathology, she insists: they ignore it or overrate it. Her discussion of the fundamentals -- dominance and recession, mitosis, DNA -- is most instructive. The book is convincing and informative, an affecting plea for counseling and responsible childbearing.