A strong familial tendency is found in both schizophrenia and
affective disorders. The importance of genetic rather than
environmental factors has been shown by reports of a high
incidence of schizophrenia in children of affected parents and
concordance in monozygotic twins, even when they are
adopted and reared apart from their natural relatives. The
same is true of manic depression. Empirical values for lifetime
risk of recurrence are available for counselling, and the burden
of the disorders needs to be taken into account. Both polygenic
and single major gene models have been proposed to explain
genetic susceptibility. A search for linked biochemical or
molecular markers in large families with many affected
members has so far failed to identify any major susceptibility
genes.
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