Quantitative Gene Action

The previously described types of gene action were all controlling characteristics that were qualitative (able to be classified). Many economically important traits in livestock are quantitative (able to be measured), such as weaning weight, egg production, milk production, or litter size. Quantitative traits are normally controlled by many pairs of genes, each with relatively small effect. They are also affected by the environment. This can be described with a simple model:

P = G + E Phenotype = Genotype + Environment

The genotype part of this model is the result of the sum of all the gene pairs that affect the trait. The gene action for the various gene pairs follows patterns that are quite similar to those involved in qualitative traits.

The following examples illustrate types of gene action for single gene pairs. In each case, an uppercase allele contributes 2 units to the trait in question. This is referred to as the additive effect. When an uppercase allele is present, there is a +2 and when two uppercase alleles are present there is a +4. The other examples illustrate different degrees of dominance. In the purely additive example, there is no other effect than that of the individual alleles. However, in this example of complete dominance, there is an additional + 2 for the heterozygote to make the heterozygote equal to the best homozygote. In partial dominance, there is an additional +1 in the heterozygote, and the heterozygote is intermediate between the homozygotes, but not exactly at the halfway point. Overdominance is the most extreme type of dominance. In this example, there is an additional + 4 in the heterozygote. The heterozygote is actually outside of the range of the two homozygotes. Quantitative genetic theory is predicated on the idea that each gene pair that influences a quantitative trait behaves in a manner that is similar to one of these pictures. Alleles have an additive effect and for many gene pairs there is also a dominance effect. In addition, gene pairs influencing quantitative traits may also interact in a manner that gives rise to epistatic effects.

Additive Complete Dominance Partial Dominance Overdominance

With the inclusion of the concepts of additive, dominance, and epistatic effects, our model can be extended:

P = G + E P = A + D + I + E Phenotype = Additive effects + Dominance effects + Epistatic effects + Environment

The symbol I is used for epistatic effects to indicate interaction (and becauseE already signifies ''environment'').

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