Relative Fitness of Wild House Mice Heterozygous for a Lethal Allele

Abstract

An attempt has been made to estimate the relative fitness of wild mice of two genotypes, those with (+/tw) and without (+/+) a lethal allele at one locus (T). Starting with a population of 67 males at birth, in which the probability of each genotype was equal, it was found to consist at 140 days of 35 +/tw and 18 +/+ animals, the difference being significant at the 0.03 level. The total number of offspring produced by the two genotypes was +/tw 1875, +/+ 991, a ratio of net reproductive efficiencies of 1.89 to 1. Of the net gain in frequency of the tw allele between the test population and their progeny, about 63 per cent was due to the segregation ratio advantage of heterozygous males and about 37 per cent to their selective advantage. Data from wild females were insufficient but did not indicate a disadvantage of heterozygous females sufficient to counteract the effect on gene frequency of the advantage enjoyed by heterozygous males. It is concluded that two evolutionary forces-male segregation ratio ad...