I examine three American researchers in the 1930s and 1940s who populate the no-man's-land of medical genetics, between the heyday of "mainline" eugenics and the medical turn in human genetics in the 1950s. In scientists' narratives, William Allan, Madge Macklin, and Laurence Snyder appear as pioneers of medical genetics and genetic education. Allan was a country doctor with an interest in heredity. Snyder, a Harvard-trained geneticist, entered medical genetics through population-genetic studies of human blood groups. Macklin came from a background in academic medicine. Allan, Snyder, and Macklin believed in a genetic approach to medicine well before genetics offered clinical benefits. Although hereditary diseases had begun to overtake infectious diseases as causes of death and illness, formal genetics offered medicine little more than a few explanatory principles. These researchers made their case by a) listing mostly hopeful potential applications of genetics to disease; b) blurring the distinction between genetics and heredity; and c) engaging in preventive genetic medicine, that is, eugenics. Examining their careers reveals some of the texture of eugenic thought in American medicine as well as the continuities between the early eugenic phase of human genetics and the professional medical genetics that today's practitioners take as the origin of their field.