Family transmission of schizophrenia: a review and synthesis.

The more recent large scale family investigations of schizophrenia are reviewed, particularly studies in which the authors have compared the rates in relatives of index cases with those of the general population. The risk of psychotic illness is increased four-fold in first-degree relatives and two-and-a-half-fold in second-degree relatives over that of the population at large. The possible genetic mechanisms are evaluated, with the conclusion that modified dominant transmission, based on a single major gene, is a plausible system.