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. 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 and 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).