The aim of this study was to determine the impact of genetic or environmental factors on the age or time of onset of IDDM by studying pairs of twins and siblings concordant for the disease.


From 404 twin pairs referred to a diabetic twin study, we selected pairs concordant for IDDM: 1) 116 identical pairs with an index twin diagnosed diabetic under age 60 years and 2) 12 identical and 12 nonidentical matched twin pairs. From 972 families referred to a population-based diabetic family study, we selected sibling pairs with IDDM: 33 pairs with an index case diagnosed diabetic under age 21 years. Twin and sibling pairs were analyzed for intraclass correlations for age and time of diagnosis.


Of twins concordant for IDDM, the age at diagnosis correlated 1) in 116 identical pairs (R = 0.94; P < 0.000001) and 2) more closely in 12 identical twins (R = 0.96, P < 0.000001) than 12 nonidentical twins (R = 0.59, P = 0.046). Of 33 sibling pairs with IDDM, the age, but not the time, of diagnosis was correlated (R = 0.53, P = 0.0016).


Correlations within pairs of twins and siblings for age, not time, at diagnosis suggest that much of the variability of the age at diagnosis of IDDM is genetically determined.

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