Allelic and nonallelic homology of a supergene family.

A model to explain the high degree of polymorphism at the major histocompatibility complex (MHC) is described. The model incorporates domain transfer between the different loci in a supergene family by either gene conversion or double unequal crossing-over. Population genetics theory is used to formulate changes in the probabilities of allelic and nonallelic gene identities and equilibrium values are obtained. The observed degree of allelic and nonallelic homology in the complex can be explained by assuming that a domain is converted at a rate of 10(-5) to 10(-6) per generation and reasonable values of other parameters. This rate of domain transfer is compatible with the observed high mutation rate at marker loci in the major histocompatibility complex.