MHC Class I
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The major histocompatability locus encodes a family of genes whose polymorphic products are expressed on most cells in the body and serve as primary "self" recognition molecules for the T cells and Natural Killer cells. MHC class I genes that are generally orthologous (identical by descent) exist in both humans and chimpanzees. However, in chimpanzees the extent of diversity of this normally very diverse group of genes appears to have been somewhat restricted, suggesting some ancestral viral infection or other selective force that caused this restriction to occur. It is interesting to note that there is evidence for a retroviral invasion of the gorilla and chimpanzee genomes several million years ago, which human ancestors appear to have been spared from.
MHC class I (and class II) genes are quite unique in human evolution, for some of them show transspecific polymorphism. That is, some chimpanzee alleles are phylogenetically closer to some human alleles than remaininng chimpanzee alleles. This causes very deep coalescence of MHC class I alleles in human, most probably caused by some kind of balancing selection. Overdominace type selection is often discussed, for heterozygous individuals may be more advantageous than homozygous ones to bacterial and/or viral infections.
Because of unusually high polymorphisms in MHC class I classic loci (A, B, and C), their polymorphism patterns can give us some clue to understand human history in terms of population size fluctuation. Because current population has huge amount of polymorphism in MHC class I loci, we can infer that our ancestors never experinces severe bottleneck (see Klein and Takahata, 2002).