MHC ALLELIC DIVERSITY AND MODERN HUMAN ORIGINS

Thirty complete coding sequences of human major histocompatibility com plex (Mhc) class II DRB alleles, spanning 237 codons, were analyzed fo r phylogenetic information using distance, parsimony, and likelihood a pproaches. Allelic genealogies derived from different parts of the cod ing sequence (exon 2, the 5' and 3' ends of exon 2, respectively, and exons 3-6) were compared. Contrary to prior assertions, a rigorous ana lysis of allelic genealogies in this gene family cannot be used to jus tify the claim that the lineage leading to modern humans contained on average at least 100,000 individuals. Phylogenetic inferences based up on the exon 2 region of the DRB loci are complicated by selection and recombination, so this part of the gene does not provide a complete an d accurate view of allelic relationships. Attempts to reconstruct huma n history from genetic data must use realistic models which consider t he complicating factors of nonequilibrium populations, recombination, and different patterns of selection.