Twenty-Five years ago the medical application of genetics consisted almost entirely of genetic counseling. It served a relatively small group of families in which a disease of known Mendelian inheritance was segregating, and usually consisted of taking a family history, estimating the risk of recurrence of the disease in particular family members, present or future, and conducting a more or less desultory discussion of the implications thereof. The extraordinary growth of knowledge and technics since then has changed the picture so radically that genetic counseling has become one of several services available in a medical-genetics center and medical genetics has . . .