Narrow-Sense Heritability

Narrow-sense heritability counts only additive genetic variance — the part that passes predictably from parent to offspring. Dominance effects depend on which two alleles happen to meet, and epistatic effects depend on combinations across loci; both are reassembled at random every time gametes form, so neither can be relied on to transmit.

The distinction is not academic. Using broad-sense heritability in the breeder's equation systematically overpredicts response, and the gap is widest for exactly the traits breeders care most about. Yield in maize carries substantial dominance variance, which is why hybrids outperform their parents and also why selecting the best-looking F₁ plants to breed from disappoints.

Estimating it requires resemblance between relatives rather than a single population's variance. Parent-offspring regression, half-sib family analysis and diallel designs all exist to separate additive from non-additive variance, and all require structured crossing rather than mere observation. That is why narrow-sense heritability is harder to obtain and more valuable once obtained.

Typical values are worth carrying as a sense-check. Highly heritable traits such as plant height, days to flowering and seed size run 0.5–0.8. Yield and other complex traits usually sit between 0.1 and 0.3. A published narrow-sense heritability above 0.9 for a yield component deserves suspicion — it usually means the environmental variance was suppressed by an unusually uniform trial rather than that the trait is unusually genetic.