A difference in the inheritance of diabetes has been shown between the families of twenty-six patients with maturity-onset type diabetes of young people (MODY) and families of thirty-five patients with classical juvenile-onset diabetes (JOD).

In the families of MODY: 1) twenty-two of twenty-six (85 per cent) propositi had a diabetic parent; 2) 46 per cent of families showed direct vertical transmission of diabetes through three generations; 3) of forty-seven tested siblings twenty-five (53 per cent) had latent diabetes; 4) the diabetic phenotype in the families was consistent, most affected individuals having a noninsulin requiring type of disease. These findings are compatible with autosomal dominant inheritance of MODY, although they do not exclude multifactorial inheritance. In contrast, in the families of JOD: 1) only four (11 per cent) of propositi had a diabetic parent; 2) three generation inheritance was found in only two (6 per cent) of JOD families, and 3) of seventyfour tested siblings eight (11 per cent) were diabetic. This difference provides further evidence of genetic heterogeneity in diabetes mellitus and indicates that there is a need for careful definition of the phenotype of diabetes in populations in which the genetics of diabetes is to be analyzed.

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