Homeostasis Associated with Heterozygosity in the Genetics of Time of Vaginal Opening in the House Mouse.

ARIATION in quantitative characters observed in long inbred strains of any Vorganism may be interpreted as being due to environmental effects. On an adequate scale, the magnitude of variation in such inbred strains should be equal, within the limit of sampling error, irrespective of the means of the different strains. In crosses between true breeding strains, the variation of the F1 generation should be equal to the variation in the parental strains. When the variation in quantitative characters is approximately uniform in the parental strains and the F1 on some scale, it may be possible to perform a genetic analysis using the methods of MATHER (1949). When no such scale is found or when the variation in F1 is markedly less than the variation in the parental strains, new assumptions must be made. Such a case was found in a genetic investigation of the time of vaginal opening in the house mouse, Mus musculus. The non-heritable variation of the F1 generation was much smaller than that of the parental strains. The non-heritable variation of the F2 generation and of the first backcross generations to males of either parental strains was also much smaller than that of parental strains, and did not differ in its magnitude from that of the F1 generation. The nonheritable variation of the second backcross generations to males of either parental strains, however, showed some increase in its magnitude, as compared to the variation observed in the F1 and the first backcross generations. Even in these second backcross generations, the magnitude of the non-heritable variation was still much smaller than that of the parental strains. Similar results have previously been reported by LIVESEY (1930) in rats, MATHER (1949, 1950) in Petunia and Primula species, RASMUSSON (1949, 1952) in sugar beets and Drosophila, ROBERTSON and REEVE (1952) in Drosophila, and DOBZHANSKY and WALLACE (1953) in Drosophila. With the exception of LIVESEY, these authors have linked their observations with the effect of different degrees of heterozygosity. I t was similarly concluded, after a statistical analysis of the present data, that the magnitude of the non-heritable variation of a generation is associated with its degree of heterozygosity. The smaller variation in the F1, F2, and the first and second backcross generations was ascribed to their greater heterozygosity as compared to that of the parental strains. To this effect of heterozygosity DOBZHANSKY and WALLACE (1953) applied the term homeostasis.