The familial incidence of certain rare diseases suggests that heredity must play a decisive part in their causation. Unfortunately, however, many characters with which one must deal in the application of human genetics to the study of disease are complex and do not clearly segregate. Unit characters cannot be identified as the basis for symptoms and physical findings unless they segregate in numerical proportions compatible with the mendelian hypothesis.
One interesting disorder of familial occurrence is the Laurence-Biedl syndrome. The cardinal characteristics of this symptom complex are obesity, genital dystrophy, mental deficiency, retinitis pigmentosa and polydactylism. The syndrome typically occurs in several members of a fraternity, while other members are normal. The familial incidence of this syndrome suggests that it is a hereditary disorder, and this presumption is strengthened by the known hereditary occurrence of mental deficiency, retinitis pigmentosa and polydactylism.
Reilly and Lisser1 were able to collect from