Heritability

Heritability is a measure of the relative proportion of genetic versus environmental factors that influence the variation of a trait (phenotype). Measures of heritability are classically divided into broad sense (H²) and narrow sense (h²) heritability. These are estimated by comparing phenotypic variation between relatives.

Broad Sense Heritability

Broad sense heritability (H²) is the proportion of variance of a phenotype that is due to genetic variation in the sample. If effects are independent the variance of a phenotype (V_P) is the sum of the variance resulting from genetic differences (V_G) and the variance due to environmental effects (V_E).

V_P=V_G+V_E

H² = V_G / V_P

For example, if the variance in germination time of a plant seed is ...

Narrow Sense Heritability

The genetic variance (V_G) of a trait is a combination of different types of genotype interactions. These can be differences due to simple dominance at a locus (V_D), epistasic interactions between loci (V_I), and additive effects both within and between loci (V_A).

V_G=V_D+V_I+V_A

Dominance and epistatic relationships are complex and depend on specific genotype configurations between parents and offspring. A simplifying assumption can be made that a large number of these effects tend to cancel out in terms of the effect on the parent-offspring correlation of a trait (while they do still contribute to the variance of a trait). On the other hand, additive effects are simple interactions; there is a total cumulative effect of a given set of alleles across loci that does not depend on the precise genotype configuration. In some cases it is easier to estimate, and more useful to focus on and use, the additive component of genetic variance. This leads to the definition of narrow sense heritability; the proportion of total variance of a trait that is due to additive genetic variance.

h² = V_A / V_P

To the degree that dominance and epistatic effects exit, it is easy to see that the narrow sense heritability will always be equal to or less than the broad sense heritability.

Things to Consider

The heritability of a trait can evolve and can be an (inverse) indirect measure of how important a trait is to a species' reproduction and survival.