Genetic and environmental influences on the secondary sex ratio in mice

Abstract

1. Over 700 litters, comprising some 4500 mice, derived from the sixteen possible types of mating within and between four inbred strains, were sexed at birth, and the parentage, parity and size of each litter was recorded. Except in a preliminary series of within-strain crosses, the first litter of each mating was not recorded. 2. There was no significant heterogeneity between the sex ratios of different litters belonging to the same mating type, parity and litter size. 3. In the main body of data inbreds and hybrids agreed well in sex ratio, giving a significant excess of males. 4. There was close agreement between the results of reciprocal crosses, but highly significant heterogeneity between the sex ratios of the six different hybrid genotypes (reciprocals combined). 5. The genetic factors responsible for the differences between hybrid genotypes appear to be those acting directly on the offspring, rather than through the male or female parent. 6. By contrast, there was no evidence of heterogeneity between the inbred genotypes. The difference in this respect between inbreds and hybrids was found to be statistically significant and was discussed in the light of the complementary phenomenon, whereby inbred strains are found to show greater within-genotype variability than interstrain hybrids. 7. The main series of inbreds had a significantly lower proportion of females and a lower mean litter size than the preliminary series from the same four inbred strains. We conclude that in our mice the female sex was the more sensitive to variations in intrauterine conditions. 8. No significant relation was detected between sex ratio and parity. 9. In the main body of data a significant rise in the proportion of males with increasing litter size was shown by both inbreds and hybrids. This result was interpreted on the assumption that intra-uterine competition was the overriding factor in foetal death. 10. No similar effect was shown by the preliminary series of inbreds. This was attributed to a reduced prenatal mortality associated with the larger mean litter size in this series. 11. In the preliminary series of inbreds, first litters did not differ significantly in sex ratio from subsequent litters. 12. On the other hand, the four strains were in significant conflict in respect of the relation between sex ratio and litter size revealed in these first litters.