HETEROZYGOSIS AND THE SURVIVAL OF YOUNG MICE IN TWO TEMPERATURES

Inbred mice of strains A, A2G, C57BL and GFF, bred in a room kept at 21° C., were crossed to give all possible F 1 types; an outbred stock was also derived from all four inbred strains. Similar matings were made among mice transferred, at the time of mating, to a room kept at ‐3° C. Mice of all types, reared at 21° C., were put in the cold room, at the age of 22 days, alone, with nesting material, for 3 weeks; controls were kept in the same conditions at 21° C. Fewer among the F 1 and outbred mice died than among the inbred; no C57BL mice survived the cold, and few of strain A. F 1 mice with one parent of strain A were less resistant than those of most other crosses. The breeding performance of females mated to males of their own strain was compared with that of outcrossed females (which bore F 1 young). More young were born per litter to some classes of outcrossed females than to the inbred mice at both temperatures, but especially at 21° C. This indicates a prenatal advantage of heterozygosis in the young. Survival between birth and weaning, at 21° C., was superior in F 1 mice borne by GFF females and, to a smaller extent, by A2G females. The outbred stock produced very large litters at both temperatures and, at ‐3° C., had a lower nestling mortality than any other group.