ANALYSIS OF QUANTITATIVE INHERITANCE OF BODY SIZE IN MICE. II. GENE ACTION AND SEGREGATION

Abstract

ISHER (1918), WRIGHT (1935) and KEMPTHORNE (1954) showed that variation F and correlation between relatives ascribable to additive effects, dominance and interaction of the genes can be partitioned. Methods of estimation of the effective number of factors have been given by FISHER, IMMER and TEDIN (1932), WRIGHT (1934, 1950), CASTLE (1921) and MATHER (1949). I n mice, BUTLER (1952) has made five crosses between strains different in body size. He found that the F, and Fz means were half way between those of the parents. backcross means were half way between that of the F1 hybrid and that of the pure-strain parent. As expected, the variances of the parental strains and the F1 hybrids were similar; however, the variance of the Fe was no larger than that of the FI. In the present study the genetic component of the quantitative variability of body size in mice has been analyzed. Among the matters that have been considered were number of loci or a t least effectively independent blocks of loci, the degree of dominance among the loci, the distribution of gene effects, and the amount of environmental contributions to the total variation. BREEDING PLANS The Large and Small strains of mice were used as the parental stocks. Their historical developments have been previously reported. There were 12 cross-bred typesused: two F1, two Fz and eight first-backcross generations. The type and number of matings, average litter sizes and designations for each of the different types of matings are given in table 1. Throughout this paper "subgroup" refers to all the females or males under each different type of mating and "genotypic group" refers to all the individuals within each pure strain or hybrid, i.e. PL, Ps, F1, Fz, BL and Bs. Maternal influence within each hybrid group was balanced by making reciprocal matings between the two parental genotypes. For instance the F1 hybrids were produced by two different types of matings, L X S and S X L. Other hybrids were similarly produced. Maternal effects will still exist between crosses, and will be considered at length below. Inbreeding was avoided as much as possible in the production of the Fz and backcross mice. There were no matings closer than second cousins or single first cousins. Therefore the effects of inbreeding were trivial and no correction is needed. Prolificness varied between mice of different genetic constitutions. The hybrid mouse was a better reproducer than the pure-strain mouse.