Gene Conversion, Linkage, and the Evolution of Multigene Families

Abstract

The evolution of the probabilities of genetic identity within and between the loci of a multigene family is investigated. Unbiased gene conversion, equal crossing over, random genetic drift, and mutation to new alleles are incorporated. Generations are discrete and nonoverlapping; the diploid, monoecious population mates at random. The linkage map is arbitrary, and the location dependence of the probabilities of identity is formulated exactly. The greatest of the rates of gene conversion, random drift, and mutation is ε & 1. For interchromosomal conversion, the equilibrium probabilities of identity are within order ε [i.e., O(ε)] of those in a simple model that has no location dependence and, at equilibrium, no linkage disequilibrium. At equilibrium, the linkage disequilibria are of O(ε); they are evaluated explicitly with an error of O(ε(2)); they may be negative if symmetric heteroduplexes occur. The ultimate rate and pattern of convergence to equilibrium are within O(ε(2)) and O(ε), respectively, of that of the same simple model. If linkage is loose (i.e., all the crossover rates greatly exceed ε, though they may still be much less than 1/2), the linkage disequilibria are reduced to O(ε) in a time of O(-ln ε). If intrachromosomal conversion is incorporated, the same results hold for loose linkage, except that, if the crossover rates are much less than 1/2, then the linkage disequilibria generally exceed those for pure interchromosomal conversion.

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