Discordant monozygotic twins; diabetes mellitus and obesity.

Identical, or monozygotic, twins offer unusual opportunities to study the role of heredity and environment in the origin of pathologic disorders. Since Galton suggested the study of identical twins as a method for the separation of hereditary and environmental influences in the development of human traits, "Gemelloiogy" has become a science in its own rights.* The striking similarities (concordance) of identical twins in physical, physiological, psychological, and pathological characteristics have always attracted attention and aroused curiosity. They were and are welcome objects of investigation for students of human inheritance. The dissimilarities (discordance) of identical twins being less impressive to the casual observer, rarely are given the consideration they deserve. Identical twins discordant in respect to a structural or functional disorder offer convincing evidence that a pathologic manifestation, observed in one twin only, cannot be determined entirely genetically. If identical twins have the same genetic endowment, their differences or discordant properties