Monozygotic twins with dissimilar phenotypes and chromosome complements.

A case history of monozygotic twins is presented. Twin A showed gonadal dysgenesis and chromosomal mosaicism (45,X0/46,XX). Twin B was a phenotypical normal female with the karyotype 46,XX. Fifteen previous reports of monozygotic twins with gonadal dysgenesis are reviewed. Dissimilar phenotypes seem to be the rule rather than the exception, and mosaicism is found in the majority of cases with dissimilar phenotype. The assumption is made that the chromosome disorder leading to gonadal dysgenesis arises postzygotically, eigher before or after the process leading to twinning.