Association of genetic polymorphisms with embryonic mortality in the chicken : III. Interactions between three loci...

Association of the egg-white genotypes of dams with the mortality of their embryos was studied in 1966 in the pure and crossbred embryos of dams of two relatively noninbred Light Sussex substrains, 6D and 6F. These had been derived two generations earlier, in 1964, by equal division of strain 6, which we previously studied in 1962; while the sires of the crosses came from two Rhode Island Red/New Hampshire substrains, 5D and 5F, similarly derived. The effects were estimated parametrically for the three loci Tf, II and III singly and in two- and three-way interaction.In the first week of incubation, Tf had an additive effect in all matings, while II (and perhaps III) had a near-additive effect in the pure matings only. There was an additive X additive interaction effect of II X III in the last six days of incubation, which was consistent over all matings except 5F X 6F. The only effect to influence the total mortality over the whole of incubation was the three-way additive X additive X additive interaction, and there was substantial agreement in this over the matings. Some triple homozygotes were superior and others inferior to the genotypes with one or more loci heterozygous.This evidence of multiple interaction between loci affecting fitness is discussed in relation to hypotheses for the maintenance of genetic variability in populations.