and Nonadditive Genetic Variation for Production

Additive and dominance genetic variances were estimated for milk and fat yield in Canadian Holsteins using deriva-tivefree REML procedures. Traits were analyzed under two sire and dam models differing in assumptions on the data structure. Within herd, dam relationships were first defined to be hierarchical within sire to estimate the two genetic variances, assuming no maternal, epistatic genetic, or other biases on the estimates. The potential impact of some of these biases was estimated as the difference between the two parental variances under a cross4assified model. Heritabilities based on sire components from both models were statistically identical and were .41 for milk yield and .32 for fat yield. Dominance genetic variance was 25% of the total variation for fat yield but was unimportant for milk yield based on approximate standard errors. However, results from the crossclassified model indicated dam components were significantly greater by 4% than sire components for fat yield Therefore, hierarchical estimates of dominance genetic variance for fat yield may be biased upward. Some form of nonadditive genetic variation for fat yield, other than that based on sire components, may require consideration in dairy cattle breeding programs.