Additive effects are tied to individual alleles, which are passed from parent to offspring. Dominance and epistatic effects arise from combinations of alleles and are not passed from parent to offspring because each gamete contains only one member of each gene pair. The additive effects are therefore of special interest as we consider how genetic improvement is made from generation to generation. The model can be altered to represent the amount of variability in the phenotypes of a group of animals with the statistical concept of variance:
This suggests that the observed variance in the phenotypes of a group of animals is created by underlying variance in the genes they possess (additive effects), the ways those genes are arranged (dominance and epistatic effects), and the environments in which they exist. It is probably important to point out that we are talking about a group of animals that exist together in the same place and time. Environment, in this context, does not mean Montana vs. Oklahoma or some other extreme environmental difference. It is the environmental variation that exists within a group of animals because of seemingly small differences in environment experienced by individual animals. There may be differences, for example, among a group of calves in the same pasture that arise from differences in date of birth, where they tended to stay in the pasture, the pathogen load to which they were exposed, or the quality of the grass. These small differences in environmental quality all contribute to the overall Var (E).
Only the additive effects are important in determining how genetic improvement is passed from parent to offspring. The proportion of the phenotypic variance that is due to additive effects (Var (A)/Var (P)) should, therefore, be an indicator of the expected rate of genetic improvement arising from selection of superior parents. This ratio is called the heritability (symbolized h2).
Estimates of heritability[1-5] suggest that traits associated with reproduction (e.g., calving interval, litter size, etc.) tend to be minimally heritable (h2<0.2). Traits associated with growth (e.g., weaning weight, average daily gain, etc.) tend to be moderately heritable (0.2 < h2 < 0.4), and traits associated with carcass merit (e.g., backfat thickness, rib eye area, etc.) tend to be highly heritable (0.4 < h2< 0.6). Highly heritable traits are those that are influenced chiefly by additive effects, whereas minimally heritable traits are those influenced mainly by nonadditive effects. Minimally heritable traits are also influenced more heavily by environmental effects, although variation in all quantitative traits has a substantial environmental component, as suggested by the fact that very few quantitative traits have heritability in excess of 0.5.
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