SOME CAUSES OF GENOTYPIC AND PHENOTYPIC DISCORDANCE IN MONOZYGOTIC TWIN PAIRS

The use of the adjective ''identical'' rather than monozygotic leads t o misunderstandings about the biology of monozygotic twinning, Most mo nozygotic twin pairs are not identical; there may be major discordance for birth weight, genetic disease, and congenital anomalies, These in dicate that postzygotic events may lead to the formation of two or mor e cell clones in the inner cell mass and early embryo that actually st imulate the monozygotic twinning event, There is also evidence that th ere may be unequal allocation of numbers of cells to the monozygotic t wins; this may have widespread implications for the cascade of develop mental events during embryogenesis, formation, and vascularization of the placenta. Large-scale zygosity testing at birth could be the templ ate for analysis of twin outcomes and their biologic causes. (C) 1996 Wiley-Liss, Inc.