MUTATION-SELECTION BALANCE UNDER GENOMIC IMPRINTING AT AN AUTOSOMAL LOCUS

I model the effect of genomic imprinting on the equilibrium allele fre quencies at an autosomal diallelic locus subject to viability selectio n and mutation. The population size is assumed to be very large; male and female mutation rates may be unequal. Different models examine cas es of the inactivation of one gene (with both complete and partial pen etrance) and of differential expression of genes according to the pare nt of origin. In the simplest cases the frequency of the deleterious a llele is approximately twice that of a dominant nonimprinting mutant, but considerably less than that of a recessive nonimprinting mutant. U nder imprinting, selection and unequal mutation rates interact: other things being equal, male-biased mutation leads to lower mutant frequen cies under maternal imprinting and higher frequencies under paternal i mprinting. I also model cases where just one allele is imprintable (an d the other not). These models allow us to predict the frequency of a failure to imprint in a normally imprinting system, as well as the fre quency of imprinting at a standard nonimprinting locus.