The single locus model of quantitative genetics is based on genotypic values being a function of two concepts, additive genetic value (breeding value), and non-additive genetic value (dominance or gene combination value).
With this model, the genotypic values are denoted as $+a$ for the A1A1 homozygote, $d$ for the heterozygote, and $–a$ for the A2A2 homozygote. The distance between the two alternate homozygotes is $2a$ (think about a number line going from values of -1 to +1 is 2 units). The value exactly half way between the two alternate homozygotes is the mid-parent value, and the amount that the heterozygote genotype differs from the mid-parent value is $d$. For convenience, the mean of the two homozygote genotypes is subtracted from all values to make the mid-parent value = 0.
Below are weights in mice associated with genotypes at a single locus.