Additive genetic effects

The additive genetic effect is an estimate of the quantitative change in a trait that is associated with substituting one allele (one genotype) with that of another allele within an interbreeding population, i.e. breeding with another kind or species. Additive effects are often calculated by genotyping and phenotyping offspring of a genetic test cross. If there are two alleles at a gene locus then the additive effect is half of the difference between the mean of all cases that are homozygous for one version of the allele (a/a) compared to the mean of all cases that are homozygous for the other allele (A/A).