HLA POLYMORPHISM IN THE HAVASUPAI - EVIDENCE FOR BALANCING SELECTION

The characterization and analysis of genetic variation at the HLA loci provides important insight for population geneticists trying to under stand the evolutionary forces that have shaped human populations. This study describes the HLA-A and HLA-B loci serotyping. and statistical analysis on an isolated Native American population, the Havasupai of A rizona. Four alleles at the HLA-A locus were identified, while eight a lleles were found at the HLA-B locus. These variants were present as 2 0 of 32 potential two-locus haplotypes, with five of the six most comm on haplotypes exhibiting high positive linkage disequilibrium. Signifi cant homozygote deficiency (heterozygosity excess) was detected both a t HLA-A and at HLA-B. This deviation from Hardy-Weinberg proportions w as not attributable to nonselective causes such as different allele fr equencies in males and females or avoidance of consanguineous matings. In addition, the distribution of alleles at both HLA-A and HLA-B was more even than expected from neutrality theory; that is, the observed Hardy-Weinberg homozygosity was only 62.4% of that expected under neut rality. These observations suggest that balancing selection is of majo r importance in maintaining genetic variation at HLA-A and HLA-B.