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Ninety-five generations of stabilizing selection for pupa weight in Tribolium castaneum resulted in a significant decrease in phenotypic variance, moderate reductions in additive genetic variance, but only slight changes in heritability for the trait. Sterility was significantly lower and the average number of live progeny per fertile mating was significantly higher in populations where stabilizing selection was practiced as compared with random selected populations. The results indicate that more genetic variability is being maintained than would be expected unless a fraction of the genes have a heterozygote advantage on the fitness scale. The reduction in phenotypic variance indicated that the populations with stablizing selection became somewhat more buffered against environmental sources of variation over the course of the experiment.

Estimation of the number of segregating genes affecting a quantitative trait in populations initiated from a cross of two homozygous lines is considered. Experimental data, for the trait in question, is assumed available on total response to recurrent selection initiated in the F2 or F3 generation, the initial additive genetic variance and the heterosis exhibited in the F1 generation. Appropriate procedures when multiplicative genetic effects are assumed are developed and reasons for assuming multiplicative rather than additive effects are indicated. These procedures were employed to estimate the number of genes affecting pupa weight in a population of flour beetles and growth in a population of mice. Estimates were 50-60 percent smaller than those obtained using familiar estimation procedures appropriate when no epistasis is assumed. However, the estimated numbers (about 200 and 100 for pupa weight and mouse growth, respectively) were still rather large.

Data on 16,557 chicks from 198 sire groups were analyzed to determine the importance of genotype-hatch interactions as sources of bias in the estimation of genetic parameters for eight-week weight in poultry. Estimates of heritability for eight-week weight obtained from a paternal half-sib correlation were .27±.04 and .29±.04 for males and females respectively when the sire component of variance was unconfounded with the sire-hatch interaction variance. If the sire-hatch interaction effects were confounded with the sire effects in an intra-hatch analysis the corresponding estimates of heritability would be .30 and .35 for males and females respectively. No attempt was made to artificially create differences in the hatch environments.The ratio of the dam-hatch interaction variance to the dam component of variance as compared with the sire-hatch interaction variance to the sire component gave a slight indication of a greater degree of importance of the interaction of non-additive genetic-environment effects than additive x environmental effects in the male progeny but not in the female progeny.