Discordant abnormalities in monozygotic twins

Congenital abnormalities are more often seen in monozygotic twins, especially with a monochorionic placenta, than in dizygotic twins. Although abnormalities are more frequently concordant in monozygotic than in dizygotic twins, discordance for the abnormalities among the former is strikingly frequent. Among 24 consecutive pairs of monochorionic (i.e., monozygotic) twins, five sets were discordant for various phenotypic traits. During the same period, among 79 pairs of dichorial twins, an abnormality was noted in only one pair, and this was discordant. The twin method has not been extensively applied to the study of the relative contributions of heredity and environment in the development of congenital disease. Its use in such studies is described and the various factors which may account for intrapair differences in isogenic individuals are discussed.