SummaryThe number of diabetic parents among 374 living parents of juvenile diabetics was no greater than expected for their age (calculated from a population survey). The number of diabetic parents who were less than 40 years of age, however, was greater than that in the population.A similar comparison of the incidence of diabetes among 535 living sibs of juvenile diabetic propositi showed a tenfold increase of diabetes over that expected and one diabetic observed among 114 offspring of the propositi was greater than expected.A preponderance of sibs with an early age at onset was found for those sibs of juvenile diabetic propositi, over that found among sibs of adult propositi.The data indicate that the predisposition for diabetes at an early age is under genetical influence which is not compatible with the single gene hypotheses of recessive, intermediate or dominant inheritance. Genetic heterogeneity between juvenile and adult diabetes is suggested and discussed. Theories of multiple genes or modifying genes for age at onset together with a single gene for the predisposition of diabetes cannot be distinguished from the data.
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