Although Mendel's laws were first rigorously tested in pea plants and fruit flies, evidence quickly mounted that they applied to all living things. Early in the 20th century, the first examples of recessive, dominant, and sex-linked inheritance were found in humans. Recessive inheritance was first revealed in alkaptonuria (1902), an enzyme deficiency that leads to cartilage degeneration, and albinism (1903). Dominant inheritance was discovered in brachydactyly (short fingers, 1905), congenital cataracts (1906), and Huntington's chorea (1913). And sex-linked inheritance was discovered in Duchenne muscular dystrophy (1913), red-green color blindness (1914), and hemophilia (1916).
Eugenicists made early contributions to our understanding of some of these disorders by constructing pedigrees of affected families. However, these disorders have easily definable symptoms (phenotypes) and are caused by single genes. Eugenicists were wrong to use simple Mendelian schemes to explain complex disorders and traits, whose phenotypes are difficult to define and which are now known to involve multiple genes or are influenced by the environment.
Eugenicists were especially concerned about hereditary blindness, because the institutionalized blind were considered a burden to society. The ophthamologist Lucien Howe conducted a study on hereditary blindness for the American Medical Association and lobbied for legislation to restrict the marriage of blind persons. Eugenicists considered epilepsy an inherited disorder, and many states sterilized epileptics to prevent its spread. This was another of the eugenicists' misinformed stands — epilepsy's causes are still not fully understood.
Today, we know of more than 5,000 single gene disorders in humans. Modern medicine views each disorder as discretely inherited; the inheritance of one disorder is unrelated to the inheritance of another disorder. Eugenics viewed disabilities as related symptoms of "bad stock." Though eugenicists believed that immorality or poor living habits were inherited, they also thought that "degenerate" traits were inherited together. Eugenicists were generally less concerned about the people affected by genetic disorders than about the threat such people posed to the purity of the national "germ plasm."