Figure 1.

Genetic effects of a single QTL with 2 alleles. The variances of QT values within genotypes can be unequal. The differences in mean values between AA and Aa genotypes need not be the same as the difference in the mean values between Aa and aa genotypes. If the mean value of the QT for genotype Aa is exactly in the middle of the mean values for genotypes AA and aa, then the 2 alleles A and a have additive effects. If the heterozygote mean is shifted toward the mean value of either of the homozygotes, then there is a dominance effect.