Direct Genetic Variance Research Articles

Maternal Effects on Weights and Backfat of Swine

This study was undertaken to estimate the genetic and environmental parameters necessary to evaluate maternal performance in swine. Records of 922 Duroc and 1726 Yorkshire pigs were grouped into families and co-variances among family members estimated. Family covariances were translated into genetic and environmental parameters by equating computed covariances to their expected compositions. Results indicated large direct genetic variances and maternal variances for birth weight in both breeds. Approximately 37% of the maternal variance in birth weight was genetic. For weaning weight in Durocs, direct genetic effects were larger than maternal effects. Maternal sources of variation were larger than direct genetic effects in both breeds for 140-day weight. The genetic portion of the maternal variance was above 80%. In Yorkshires the individual environmental variance of backfat thickness was large. Of the remainder, the maternal contribution was estimated to be 3.5 times as large as the direct genetic contribution. In all cases the genetic correlation between direct and maternal effects was large and negative, indicating an apparent genetic antagonism between direct and maternal effects. The estimates of parameters in this study indicate that in these populations breeding values for either direct effects or maternal effects for the weights studied could be improved by selection. However, the negative genetic correlation between these two types of effects would make simultaneous improvement difficult and thus frustrate attempts to improve the performance of these traits by selection.

Covariances Among First-Lactation Milk Records of Cousins

Covariances among first-lactation milk records expressed as deviations from herd-mate averages of Holstein cows related as cousins of varying degree, as daughterdam, as full and maternal sibs, and as aunt-niece of varying degree were examined to determine whether genetic maternal effects are important. The multiple regression of the expected coefficients of additive direct, additive maternal, dominance direct, and dominance maternal genetic variance components; and additive direct by additive maternal and dominance direct by dominance maternal genetic covariances on the 24 estimated covariances between pairs of relatives gave a multiple correlation coefficient of .95. The regression of actual covariance on the coefficient of additive direct genetic variance gave a correlation of .92. These results suggest for these data that only additive genetic effects are important for this trait. Additive genetic variance accounted for 38% of total variance. The intercept of the regression line indicated that the average environmental covariance among pairs of relatives accounts for 2 to 3% of the total variance.