Cytogenetic investigation in twins with manic-depressive disorders (22 monozygotic and 27 dizygotic twin pairs).

Chromosome examination was made in 22 monozygotic and 27 dizygotic twin pairs of whom one or both of each pair suffered or had suffered from manic-depressive disorder. We found a significantly higher frequency of chromosome variations among dizygotic twin pairs than was expected from population studies, but not in monozygotic pairs. There was no association between the chromosome variations and manic-depressive disorders. We found no greater intra-pair correlation in monozygotic twins compared with dizygotic twins as regards hypodiploidy, hyperdiploidy and unstable chromosome aberrations, which indicates that the aetiology of such aberrations is mainly of exogenic nature.