I'm trying to get a better understanding of quantitative genetics by reading Lynch and Hill 1986. I'm getting stuck with some notation that's used right at the beginning referring to the mutant and WT alleles at a given loci.
2ai is the genotypic value for WT homozygous
(k+1)ai is the genotypic value of the F1 heterozygous
0 genotypic value for the non-existent "mutant" homozygous
From my understanding, a is the additive genetic effect and k is the dominance coefficient. I understand that 2ai is the additive genetic effect at this particular loci which moves this trait away from the population mean for this trait.
Can someone explain the (1+k)? I understand that there could be dominance between the new and old allele, but I don't understand why 1 needs to be added to the dominance coefficient.