Deficit of HLA homozygotes in a Caucasian isolate.

Deficits of HLA-A, -B homozygotes observed many years ago in two inbred populations suggested negative selection against HLA homozygotes. To determine whether a similar deficiency would be observed in the S-leut Hutterites, a well-characterized Caucasian isolate of European ancestry, and to determine whether selection operated at the allele, locus, or haplotype level, observed and expected numbers of homozygotes were compared in 852 adult Hutterites. Deficits ranging from 11% to 24% were observed for all five loci examined (HLA-A, -B, -C, -DR, and -DQ). However, these deficits were secondary to, and almost completely accounted for by, a 64% loss of individuals homozygous for the haplotype. There was no evidence of deficits affecting only a single allele or locus. The data indicate strong negative selection against HLA homozygotes. This could be due, at least in part, to decreased fecundability among couples sharing HLA-DR. However, these data suggest that additional selective factors acting at the level of the haplotype also operate in this population.