This paper focuses on sickle cell anemia and thalassemia as case studies of genetic disease in America. Before the 1950s, these diseases were perceived by many physicians as closely related (indeed, by some as indistinguishable). Sickle cell anemia was defined by most American physicians as a Mendelian dominant disorder specific to African-Americans. As such, it could be 'spread' by any individual parent 'carrier' through reproduction. This view of the disease fed into (and was supported by) prevalent social concerns about miscegenation and, more generally, the dangers inherent in 'negro blood'. A particularly thorny problem for American physicians was how to explain cases of 'sickle cell anemia in white patients'. The paper examines how views about race, blood, and Mendelian genetics informed broader debates about the nature of hereditary disease and social relations in America from 1910 to 1950.