Partly dominant transmission of schizophrenia in Iceland.

Voluminous data on the familial distribution of schizophrenia in Iceland were evaluated in terms of a partly dominant genetic mechanism. The risks were found in most instances to be proportional to the probability of harbouring the proposed gene, but penetrance rates, which generally are around 25%, appear to be elevated when additional risk factors are present. This occurs in individuals with multiple ill relatives, in offspring of two schizophrenic people, or in monozygotic co-twins. Excessive risks also occur in families with unusually severe disease.