<P> An upper case H is used to denote broad sense, and lower case h for narrow sense . </P> <P> For traits which are not continuous but dichotomous such as an additional toe or certain diseases, the contribution of the various alleles can be considered to be a sum, which past a threshold, manifests itself as the trait, giving the liability threshold model in which heritability can be estimated and selection modeled . </P> <P> Additive variance is important for selection . If a selective pressure such as improving livestock is exerted, the response of the trait is directly related to narrow - sense heritability . The mean of the trait will increase in the next generation as a function of how much the mean of the selected parents differs from the mean of the population from which the selected parents were chosen . The observed response to selection leads to an estimate of the narrow - sense heritability (called realized heritability). This is the principle underlying artificial selection or breeding . </P> <P> The simplest genetic model involves a single locus with two alleles (b and B) affecting one quantitative phenotype . </P>

An estimate of the proportion of the phenotype variation due to the genes