The family history of diabetes in the first-degree relatives of 1,307 patients at the diagnosis of clinical diabetes is compared with a similar family history in 859 control-group subjects. The results are matched for siblings and parents in different age-groups, and are also compared with the known rate of clinical diabetes at different ages. Under 30, compared with a nondiabetic control, a diabetic at diagnosis is between 12 and 24 times as likely to have a similarly affected sibling. The corresponding rate for those aged 70 or over, is only one and a half times. The results suggest that only diabetes of early onset has a strong genetic background.
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Johnston et al. (1965) studied this question.