The additive component of heritability
The heritability of traits is determined by the fraction of genetically determined variability in the given trait in the total variability of this trait, i.e. in the variability determined both genetically and nongenetically (by the effects of the external environment). Heritability in the narrow sense of the word expresses the fraction of the additive component of genetically determined variability in the total variability of the given trait. Additive variability is the component of the variability that is additive in its effects. If allele A of one gene acts, on an average, in its carriers to increase their body weight by 10% and allele B of another gene acts, on an average, in its carriers to increase their body weight by 5%, and if this is an additive component of the variability in both cases, then the carriers of alleles A and B will be, on an average, 15% larger than the carriers of other alleles. If this increase is smaller or greater than 15%, then nonadditive variability is involved.