heritability

Meaning: Heritability is the proportion of variation in a trait, within a particular population and environment, that is due to genetic differences among individuals. It describes variation, not how much genes contribute to a trait in one person. Broad-sense heritability counts all genetic variance; narrow-sense counts additive effects alone. Less environmental variation raises heritability, though the genes are unchanged.

Equation: H² = VG / VP, where VG is genetic variance, VP is total phenotypic variance, and VP = VG + VE (VE is environmental variance). Variances carry the trait's unit squared, so H² is unitless, between 0 and 1. Narrow-sense: h² = VA / VP, where VA is additive genetic variance.

On the MCAT: Passages use twin and adoption studies. Higher concordance in monozygotic than dizygotic twins, or adoptees resembling biological more than adoptive parents, points to higher heritability. Predict that a more uniform environment raises heritability and a more varied one lowers it.

Common confusion: High heritability does not mean a trait is fixed, and it does not explain differences between groups. Height is highly heritable within a population, yet average height can rise across generations with better nutrition.