Additive genetic effects are the contributions to the final phenotype from more than one gene, or from alleles of a single gene (in heterozygotes), that combine in such a way that the sum of their effects in unison is equal to the sum of their effects individually. Genetic effects that are not additive involve dominance (of alleles at a single locus) or epistasis (of alleles at more different loci).
References
Additive genetic effects Wikipedia(Text) CC BY-SA