Among 242 sets of twins collected from 34,072 Joslin Clinic records from 1949 to 1966, there were 104 sets with both members alive. Of these, forty-seven sets were examined and an additional forty-three sets responded to a detailed questionnaire. A high concordance for overt diabetes was found in monozygotic sets in which the proband was age forty or greater at time of diagnosis. In the forty-seven sets examined, there were no cases of overt diabetes in the dizygotic twin siblings of a proband who was age forty and over at time of diagnosis of overt diabetes. Surprisingly, however, a high rate of chemical diabetes was diagnosed in the dizygotic twin siblings, an observation implying an etiology for diabetes mellitus more complex than inheritance by a single recessive gene.

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