A Difference in Skeletal Type between Reciprocxl Hybrids

IFFERENCES between reciprocal hybrids of strains of various animals D are not uncommon. In the house mouse an example is the incidence of spontaneous mammary cancer in crosses between “ high incidence ” and ‘* low incidence” strains (MURRAY and LITTLE 1935). When it may be assumed that the cause of the differences between the reciprocal hybrids is not genetic, as in matings between inbred strains, new possibilities are presented for analyzing non-genetic causes of morphological and physiological variation. The purpose of this paper is to describe reciprocal crosses between two highly inbred strains of mice, differing in skeletal type, which resulted in F1 hybrids showing a significant tendency to resemble the skeletal type of the strain of the female parent. Matings were made, reciprocally, between these two strains for the purposes (1) of determining the nature and estimating the number of pairs of genetic factors necessary to account for the difference in skeletal type between the strains, and (2) of studying the effect, if any, of several environmental variables on a genetically homogeneous, although hybrid, group of F1 animals. This paper deals with one aspect of the second purpose. The difference in skeletal type between the reciprocal hybrids represents a major finding in the study of the environmental variables. The difference suggests that some elements of the environment, furnished by the ova or by the mothers to the developing embryos, differ in the two strains, and that the nature of these elements may be identified by appropriate experimental techniques, such as for example by transplantation of ova or of ovaries.