The maintenance of polymorphisms at two loci in house mouse (Mus musculus) populations

To determine the roles of mutation, migration, and selection in maintaining genetic variability in populations of house mice (Mus musculus), stochastic models based on characteristics of mouse populations inhabiting corn cribs in southwestern Ontario and the t allele were developed. Two sets of models were examined. One involved selection against t/t mice and a migration rate of 0.05 to 0.10 (low migration model) whereas the other involved selection against both t/t and +/t genotypes and a migration rate of 0.33 (high migration model). Both models could account for the t allele frequencies observed in natural populations. Similarly both models explain the frequencies observed at a second polymorphic locus, the Hbb locus which controls the beta chain of the hemoglobin molecule, provided strong selection favoring the Hbb heterozygotes is incorporated. Without such selection pressure rapid extinction of one of the alleles at this locus occurred. A stabilizing force such as selection is considered necessary for the hemoglobin polymorphism observed in the populations under consideration. Evidence supporting the high migration model as the more realistic is also presented.