The idea that a particular physical feature, either normal or abnormal, is hereditary is probably as old as our species itself.

However, as has been noted by many other writers, tracing the origins of an idea is a formidable, if not impossible, task. Clearly, the concept of “like begets like” found a practical expression in the early domestication of animals; breeding stock was chosen on the basis of favorable traits.

This essay traces the development of the concept of heredity and, in particular, shows how that devel­opment has shed light on the host of hereditary and genetic diseases we have come to recognize in hu­mans. It begins with a brief discussion of some basic concepts and terms, which is followed by an outline of the heuristic model of genetic transmission that has come to be the standard of modem medical genet­ics. Once this groundwork is in place, the history of the study of human genetic disease is developed from the earliest records, through the birth of medi­cal genetics, to the molecular era. Naturally, a de­tailed narrative of this history would require several volumes. Therefore, some events and ideas have been omitted or treated only cursorily.

The most recent development in the study of hu­man genetic diseases is traced through three specific examples. Each of these represents a microcosm of the development of medical genetics. Our under­standing of the first of them, sickle cell anemia, represents a triumph of the molecular model of hu­man disease. The discovery of the second, Down syn­drome, reveals the role in medical genetics of the cytogeneticist, who studies chromosomes. The third, kuru, exemplifies a case in which the expectations of modem medical genetics led initially to an errone­ous conclusion, although that failure led eventually to spectacular new knowledge. This essay closes with a brief assessment of the future development of medical genetics in the molecular era.