narrow-sense heritability
Narrow-sense heritability, written h², measures only the part of trait variation that parents reliably hand down: the additive contribution of their alleles. It is the share of differences that behaves predictably from one generation to the next, like the part of a recipe you can count on tasting the same each time because the ingredients simply add up.
The reason for singling out the additive part is practical. When a sperm and egg form an offspring, each parent passes on single alleles, not whole genotypes — the special pairings and interactions that gave a parent its exact phenotype get shuffled and broken up. Only the additive effects, where each allele contributes the same nudge regardless of its partners, survive that shuffling and so predict resemblance between parent and child.
This makes h² the number breeders and quantitative geneticists reach for first, because it predicts how much a population will respond to selection: pick the largest animals to breed and the average shifts in proportion to h². A low narrow-sense heritability warns that selection will move slowly, even if the trait is strongly genetic in the broad sense. As always, the figure is specific to one population and environment.
Dairy breeders rely on the moderate narrow-sense heritability of milk yield: by mating bulls whose daughters produced the most milk, each generation lifts the herd average by a predictable amount.
Additive variance drives the response to selection.
Formally, h² equals additive genetic variance divided by total phenotypic variance. The breeder's equation, response equals h² times selection differential, makes this number the workhorse of animal and plant improvement.