The model of phenotypic variance that we’ve just developed can be used to address the question of how much of the phenotypic variance in a characteristic is due to genetic differences. Broad-sense heritability (H²) represents the proportion of phenotypic variance that is due to genetic variance. It is calculated by dividing the genetic variance by the phenotypic variance:

The symbol H² represents broad-sense heritability because it is a measure of variance, which is given in units squared.

Broad-sense heritability can potentially range from 0 to 1. A value of 0 indicates that none of the phenotypic variance results from differences in genotype and all of the differences in phenotype result from environmental variation. A value of 1 indicates that all of the phenotypic variance results from differences in genotype. A heritability value between 0 and 1 indicates that both genetic and environmental factors influence the phenotypic variance.

Often, geneticists are more interested in the proportion of the phenotypic variance that results from the additive genetic variance because, as mentioned earlier, the additive genetic variance primarily determines the resemblance between parents and offspring. Narrow-sense heritability (h²) is equal to the additive genetic variance divided by the phenotypic variance:

TRY PROBLEM 15