Characteristics of a multiplex IDDM sample: unexplained differences with other samples.

Characteristics of a multiplex sample of families with insulin-dependent diabetes mellitus (IDDM) are studied and contrasted with similar characteristics in other, more conventionally sampled data sets. Some characteristics remain consistent with earlier observations including the high frequency of human leukocyte antigen (HLA) DR3,4 in affected individuals and the greater than expected percentage of HLA haplotype sharing among affected sib pairs. In other respects, however, differences are seen between this sample and others. "Control" haplotypes, i.e., those not transmitted to the first affected offspring, had a higher frequency of DR3 and DR4 than expected, and a rather high frequency of affected parents was observed. Differences between the first affected and later affected offspring reported in other samples were absent from these families. No effect of the sampling scheme and the resulting distribution of parental phenotypes could be shown to explain this difference.