Effect of gene conversion on polymorphic patterns at major histocompatibility complex loci.

The pattern of polymorphisms at MHC loci was studied by computer simulations and by DNA sequence analyses, with special reference to synonymous and non-synonymous divergence among alleles. A model of overdominance plus short-term selection was simulated for a gene family with allelic and non-allelic gene conversion. It was found that the ratio of non-synonymous-to-synonymous divergence among alleles decreases as the non-allelic conversion becomes more frequent. On the other hand, the ratio of synonymous divergence at the antigen recognition site to that at non-coding region increases by non-allelic conversion. By comparing such a pattern with the observed values of mammalian genes, it is suggested that non-allelic conversion is less frequent in human genes than in genes of other mammals. A quantitative analysis on the patchwork pattern was attempted by using identity excess among nucleotide sites of a gene. Comparison of simulation results with DNA data analysis shows that the rate of allelic conversion is high.