Patterns of occurrence of a disease in a family pedigree or genealogy can suggest transmission of a mutation closely related to that trait. Hemophilia in male descendants of Queen Victoria is a classic case, in which female descendants can be carriers of the mutation: pedigree diagram.
During the heyday of eugenics in the early C20, the same method was used to suggest transmission of genes related to much less specific traits, such as, intellectual renown in the Darwin-Wedgewood-Galton family tree: pedigree (see additional images)
Or feeble-mindedness in the Kallikak family, the subject of a 1912 book by H. Goddard: pedigree. Goddard’s conclusion, that the reproduction of people with such hereditary feeblemindedness should be limited, was soon institutionalized in compulsory sterilization legislation in the USA and then in other countries (see wikipedia entry on rise and decline of eugenics).
(Introduction to this series of posts)