Models based on parental general combining ability (GCA) effects, which are a function of additive genetic effects, were introduced in Chapter 4. In this chapter we introduce specific combining ability (SCA) effects that result from dominance effects (interactions among alleles within individual loci). The mean genotypic value of offspring from a particular cross may deviate from the value expected based only on the population mean and the sum of the parental GCA effects (Falconer and MacKay 1996). This deviation is the specific combining ability for that cross. We can define the mean genotypic value (GAB) for the full-sib family produced by crossing parents A and B as the sum of the overall mean (μ), the GCAs of the two parents and the SCA value: