This investigation was undertaken to determine the relative importance of additive, dominance, epistatic, maternal and sex-linkage effects for reproduction and growth traits in mice.

Two selected lines of mice, extremes in phenotype for gain from 21 to 42 days of age, were used in a triple test cross design. The high line had completed 24 generations of selection for increased postweaning gain while the low line was three generations behind. Generations 1 and 2 generated F₁ and F₂ lines, generations 3 and 4, replicates, were the backcross of F₂ males to high, low and F₁ females. Reproductive traits measured on each dam were number born per litter, mortality of young from birth to 5 days, number of days from exposure of male to littering and 12-day litter weight. Height traits measured on each offspring were 12-, 21-, 42- and 56-day body weights. Three gain traits were computed for each individual: 12-21, 21-42 and 42-56 day gain.

Analyses of the data were of two levels. The first-order tests used components of means to determine adequacy of various genetic models and obtain estimates of their parameters. Four genetic models tested were: (1) containing additive-dominance effects only, (2) adding non-allelic interactions, (3) containing additive and dominance autosomal and sex-linked effects and (4) containing additive and dominance autosomal and maternal effects. Second-order tests using components of variation were used to detect epistasis and estimate additive, dominance and environmental sources of variation.

Results showed that additive effects were important for reproductive traits except littering time and mortality of young from birth to 5 days. Body weights at 12, 42 and 56 days of age in both sexes had significant additive variance components. Percent contributions of additive variation to the phenotypic expression for body weight were 17.4 and 9.5% of 12-day weights, 33.8 and 5.4% of the 42-day weights, and 32.9 and 29.5% of 56-day weights for males and females, respectively. Gain from 21-42 days of age showed significant additive genetic variation, 20.9 and 12.0% in males and females, respectively. Estimates of dominance components for littering traits were all zero. A few weight traits showed dominance to be a significant source of genetic variation but percent contribution to phenotypic variation was quite small. Traits for which epistatic interaction were important genetic influences were littering time, mortality from birth to 5 days and 12-day individual weight for males and females. Weaker epistatic interactions were found for body weights at 21 and 56 days of age. Maternal effects, both additive and dominance, were large genetic contributors to the phenotypic variation for littering and weight traits. For all reproductive traits and 12-day individual body weight, estimates of maternal components were as large as the additive component, but for 21-, 42- and 56-day weights and 12-21, 21-42 and 42-56 gain, additive effects are much larger. Sex-linkage is not an important genetic force in growth traits of mice.