The level of deep branching at X-linked loci indicates loci were close to fixation or fixed at the end of the human population bottleneck 100,000 to 150,000 years ago. The HLA-DR locus represents a major exception to this observation. Based on distribution of major groupings in the human population it is possible to assert that more than a dozen major variants survived the population bottleneck. This observation is supported by the concept of a heterozygous selection coefficient operating on the HLA-DR, and at the HLA-DRB1 locus to a greater degree relative to HLA-DQB1 and HLA-DPB1. Most of the HLA alleles currently present in the human population can be explained by gene conversion between these ancient ancestral types, some that persist into the extant population.