Topic 11 - Heritability of Quantitative Traits

What is Heritability

It is the proportion of phenotypic variance due to genetic factors in a given population and environment. It is the ratio of genetic variance to phenotypic variance. Two metrics of heritability:

• Broad sense (H2)

• Narrow sense (h2)

Both range from 0 - 1:

• 0 = all phenotypic variance in a trait is due to environmental conditions

• 1 = all phenotypic variance in a trait is due to genetic factors

Broad sense Heritability in Humans

Used to measure the relative importance of genetic and environmental effects on a trait, but it represents all genetic variance as a single value.

Very high broad sense heritability means that > 80% of variation in a trait is due to genetics.

High broad sense heritability means that 50 - 80% of variation in a trait is due to genetics.

Moderate broad sense heritability means that 20 - 50% of variation in a trait is due to genetics.

Narrow sense Heritability in Humans

Selection/adaptation can only operate on additive. Narrow sense heritability only uses additive, which means that it is the most useful heritability estimate with respect to adaptation and selective breeding.

High narrow sense heritability means that > 60% of variation in a trait is due to additive genes.

Moderate narrow sense heritability means that 25 - 60% of variation in a trait is due to additive genes.

Low narrow sense heritability means that < 25% of variation in a trait is due to additive genes.

Breeder’s Equation

It is used for predicting the response to selection.

R = h2 x S

Response to selection = narrow sense heritability x the selection differential (difference between the mean of the base population and the selected group).

What happens when selection takes all beneficial additive alleles to fixation

Selection depletes the additive genetic variation (VA) as alleles go to fixation. Therefore, h2 also declines. Heritability is a statistical property of a population, not a general fact about the genetic basis of a phenotypic trait.