Abstract
Although genetics is central to medicine today, physicians routinely used an alternative system of Medical Heredity well into the 20th century. This reflected general medical experience, and applied to non-inherited disorders as well. There were six major overlapping and interacting components: 1. Heredity, a transmitted force; 2. Diatheses, or predispositions; 3. Polymorphism, or variability; 4. Neuropathic and psychopathic inheritance. 5. Degenerate heredity, or degeneration, a progressive weakening of heredity with transmission. 6. Blastophoria, a prenatal weakening of heredity from adverse influences. Also included were now outmoded concepts of biotypes, atavism, and stigmata of degeneration. Physicians noted familial disorders in siblings, hereditary conditions in multiple generations, and even sex-limited inheritance, seemingly anticipating Mendel. However, this system was better suited to common types of inheritance than to rare genetic disorders. It was supported by eugenics until eugenics was discredited in the late 1940s, and vanished by the late 1950s as molecular biology validated Mendelian genetics and undermined Medical Heredity, while antibiotics and vaccinations reduced support from experiences with infectious diseases.