Some models of gene conversion for treating the evolution of multigene families.

A simple model of gene conversion presented earlier has been compared with a more general model under the assumption of no bias. The former is based on a simple concept that, by conversion, a gene (or a segment of a gene) is transferred from one locus to another in a multigene family. Intrachromatid symmetric conversion may result in reciprocal exchange of genes within a multigene family, and the previous theory of identity coefficients needs modification when it occurs. The relationship of various parameters of the two models was made clear in formulating the transition equations of identity coefficients. As an example of extending the analyses, a model of preferential conversion that is relevant to the observed uneven polymorphisms of class I genes in the major histocompatibility complex has been investigated. It differs from the biased conversion, in that directional process is through location of genes on the chromosome. It is shown that the model satisfactorily explains the observed data, even though the effect of preferential conversion is difficult to distinguish from that of natural selection.