Heritability is the proportion of the observed variation in a trait, in one population and one set of conditions, that is explained by genetic differences. It runs from 0 (all differences are environmental) to 1 (all are genetic). Broad-sense heritability (H²) counts all genetic effects; narrow-sense (h²) counts only the additive part, which is what predicts response to selection.
In practice
High-heritability traits respond quickly to selection: pick the best parents and the offspring follow. Flowering time and the cannabinoid synthase type behave like this. Low-heritability traits (yield per plant is the classic) are dominated by environment and plant position, so selecting the biggest plant in a tent often picks the luckiest spot, not the best genes.
That is why good breeders grow candidates in uniform conditions, repeat them as clones, and judge parents by their offspring.
Units and formula
H² = VG / VP and h² = VA / VP, where VP is total phenotypic variance, VG genetic variance and VA additive genetic variance. Response to selection: R = h² × S (the breeder's equation), where S is how far the selected parents are above the population mean.
Don't confuse with
- Inherited: a trait can be fully genetic yet show zero heritability in a uniform population, because there is no variation left.
- Fixed number: heritability belongs to a population and an environment; change either and it changes.
