One way to estimate the relative contribution of genes to a trait is through ‘variance component analysis’ in related individuals. In this approach, the total variance in a quantitative, or measured, phenotype is divided into its genetic and environmental components. The proportion of the total phenotypic variance attributed to genetic factors (i.e., genetic differences between individuals) is referred to as the heritability of the trait. The genetic variance can be further divided into the variance due to additive genetic effects, to non-additive genetic effects (e.g., dominance, recessiveness, epistasis), as well as be assigned to autosomes or sex chromosomes. The proportion of the variance due to additive genetic effects is referred to as narrow heritability (h2); the overall proportion of genetic variance is referred to as broad heritability (H2). The theoretical basis for heritability estimates and derivation of the individual variance components has recently been reviewed37. The heritabilities of many human traits have been estimated, although most studies are limited to estimates of narrow heritabilities in combined samples of males and females (for examples, see refs.38-40).
