Revision summary
Heritability is a population-and-environment-specific share of phenotypic variance. Broad H² includes all genetic variance; narrow h² includes additive variance. Fisher's variance partition underlies the concept. Twin, adoption, pedigree and parent–offspring designs estimate familial resemblance. Equal-environment assumptions and gene–environment correlation limit twin estimates. High heritability does not mean immutability or explain group differences.
Model answer
Introduction
Heritability is the proportion of observed phenotypic variance in a specified population and environment attributable to genetic variance. It does not measure how genetic an individual's trait is, nor does a high value make a trait unchangeable.
Body
Concept and estimation
Quantitative genetics partitions phenotypic variance as V(P) = V(G) + V(E) + interactions. Broad-sense heritability, H² = V(G)/V(P), includes additive, dominance and epistatic variance. Narrow-sense heritability, h² = V(A)/V(P), retains additive variance and best predicts response to selection; R. A. Fisher supplied the variance framework.
Estimation uses resemblance among relatives:
- parent–offspring regression and sibling correlations estimate additive familial resemblance;
- classical twin studies compare monozygotic with dizygotic twins, often using h² ≈ 2(rMZ − rDZ) under restrictive assumptions;
- adoption studies separate resemblance to biological and rearing relatives;
- pedigrees and modern genomic relatedness methods estimate variance components in large samples.
Height is highly heritable in many well-nourished populations, yet mean stature can rise rapidly with nutrition. IQ estimates similarly vary by age, class and environment. Twin estimates assume comparable environments and may be inflated by gene–environment correlation, assortative mating or shared prenatal conditions. Population stratification can also distort genomic estimates.
Thus heritability is local to population, age, environment and measurement. It says nothing directly about between-population causes.
Flow diagram
flowchart TD VP[Phenotypic variance] --> VG[Genetic variance] VP --> VE[Environmental variance] VG --> VA[Additive] VG --> VD[Dominance and epistasis] VA --> H[Narrow h squared] VG --> B[Broad H squared]
Conclusion
Heritability converts family resemblance into a variance estimate. Used carefully, it clarifies polygenic variation; used as destiny, it revives biological determinism. The anthropological answer must always specify whose population and which environment.
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Does 80% heritability mean 80% of a person's height is genetic?
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Can heritability change?
Yes. Alter the environment, age range, population or measurement and the variance ratio can change.
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