Human genetics is regarded by many clinicians as a vague discipline at the periphery of the main stream of medicine, contributing little to medical science or practice. This is related to a dearth of experimental tools and techniques which can be effectively used to study hereditary factors in human disease and to lack of familiarity with those which are available. Controlled animal and plant breeding experiments have given us understanding of genetic mechanisms but may not be directly applied to the human species. Since controlled human breeding experiments are impossible, the human geneticist has had to rely on family pedigree studies. These are of limited value because they are time-consuming, because of difficulty in interpretation, and because results are frequently inconclusive. Such studies are frequently impossible because of social or economic factors. During the past 5 years evidence has been found of associations between the ABO blood groups and various