In quantitative genetics, one is interested in the extent to which variation in certain characteristics is heritable. Heritability is expressed in terms of the proportion of the variance of a trait in a population that can be attributed to genetic differences. This genetic variance component can be estimated in, for example, the classical twin design (Boomsma et al. 2002a) in which the covariance structures of monozygotic and dizygotic twins are compared.
