5.6 Simplify the genetic model

The G in our model P = G + E is quite complex as it has a number of underlying components. This can be modelled as:

Genotype = additive effect + dominance effect + epistatic effect

Or G = A + D + I

To start at the back: the epistatic effects indicate that there are genes interacting with each other. This is, for example, the case if one gene needs the product of another gene to come to expression, resulting in so-called gene-pathways. Expression of one gene thus depends on the allele combination in another gene. The dominance effects indicate that expression of the gene itself depends on the allele combination in that gene. Two recessive genes will result in a different expression from one recessive and one dominant allele. The additive effects indicate the effect of the gene without the dominance and epistatic effects. So irrespective of the allele combinations of the gene itself or of other genes. What remains are effects that you can add up.

Definitions

The genetic component consist of three underlying effects:

The epistatic effect: interaction between genes
The dominance effect: interaction between alleles of the same gene
The additive effect: everything that is left over after correcting for the interacting effects

In variance component terms the genetic variance can thus be written as

σ²_G = σ²_A + σ²_D + σ²_I

To be precise, this equation should be extended by “+ 2cov_A,D + 2cov_A,I + 2cov _D,I”, however these co-variances are zero by definition and are, therefore, left out of the equation.