Where a number of genes together act on the same trait, they can do this in three ways:
- additively or
- dominantly or
For self-pollinators, most breeding (except for F1 hybrids) additive variance is the most important of these, because dominant gene action is irrelevant where genes are homozygous, and epistasis is generally a minor factor. But for cross-pollinators, both additively and dominantly acting genes are equally important, because many genes will be in the heterozygous state.