In contrast, narrow-sense heritability reflects only the contribution of additive (allelic) genetic effects to a heritability estimate and is defined as h2 =σ2
a/σ2p. This additive genetic effect is only a portion of the total genetic effect and it refers to the degree of the phenotypic variance predictable by the additive effect of allelic substitutions, such that the genetic contribution for a heterozygotic pair of alleles is halfway between that of the two homozygotic pairs. As molecular genetic experiments involve mapping allelic variation to trait variance, narrow-sense heritability is the focus of most modern genetic investigations, including imaging genetics studies.