A STUDY OF SIZE INHERITANCE IN THE HOUSE MOUSE: II. ANALYSIS OF FIVE PRELIMINARY CROSSES

Abstract

Five crosses were made between mice of different body size, and over 2000 mice were raised in the F₁, F₂, and backcross generations. The body weight at 60 days after birth was used as the criterion of size. The male means were always larger than the corresponding female means and the difference in weight between the two sexes increased progressively with body size. Litter size and sequence had no effect on body size. The adequacy of the gram scale was tested with inconclusive results which indicated that in at least two crosses some other scale should be used. Log-grams were substituted for grams and gave a good fit in cross No. 3 but not in crosses Nos. 1 and 2. The evidence from selection experiments, environmental variability, and sex differences in size indicate that on a gram scale at least part of the factors which affect body size are proportionate rather than additive in nature. In all five crosses the F₁and F₂means are intermediate between the parents. The backcross means are halfway between the F₁and the respective parent. Only one cross showed increased size in the F₁which might be interpreted as due to heterosis. Reciprocal crosses gave significantly different results and the dissimilarity was carried over into the next generation. This difference was attributed to the environmental effects of female body size. As expected, the variances of the P₁'s and the F₁were similar but, contrary to expectation, the F₂variance was no larger than that of the F₁. Litter size showed a different type of inheritance. One cross between P₁'s with mean litter sizes of 5.1 and 10.2 gave an F₁mean of 13.2 young. This was tentatively interpreted as dominance of large litter size and hybrid vigor allowing more embryoes to reach parturition.