Relative Fitness of Wild House Mice Heterozygous for a Lethal Allele

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Abstract
An attempt is made to estimate the relative fitness of wild mice of 2 genotypes, those with (+/tw) and without (+/+) a lethal allele at one locus (T). A population starting with 67 males at birth, in which the probability of each genotype was equal, 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 2 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% was due to the segregation ratio advantage of heterozygous males and about 37% 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 2 evolutionary forces[long dash]male segregation ratio advantage and selection[long dash]both act to increase the frequency of such lethals in populations. Other forces probably act in the opposite sense to produce the equilibrium values of these alleles. These forces remain to be identified.