Assume that two gene loci (A and B) influence a quantitative trait. At each gene locus, there are two alleles (A1 and A2, B1 and B2).
At the A locus, the two alleles act additively where the A1A1 genotype (value of +20) has a phenotype that is 10 units higher than the heterozygote (value of +10), and 20 units higher than the A2A2 genotype (value of 0). This same gene action and genotype values also exist at the B locus, and, the effects of the A and B loci genotypes are additive.
The aggregate or overall genotypes are cumulative across the gene loci for alleles that affect the trait.