Heritability Calculator

Heritability is a population-level statistic that describes the proportion of phenotypic variation in a trait that can be attributed to genetic variation among individuals. It is one of the most important and most misunderstood concepts in quantitative genetics, animal and plant breeding, and behavioral science. This heritability calculator lets you estimate both broad-sense heritability (H²) and narrow-sense heritability (h²) directly from variance components, and it also derives realized heritability from a selection experiment.

The central idea is that any measurable trait — height, milk yield, seed weight, blood pressure, or grain protein content — varies across a population. That total variation, called the phenotypic variance (Vp), can be partitioned into a genetic portion and an environmental portion. Heritability simply asks: of all the differences we observe between individuals, what fraction is driven by their genes rather than their surroundings? A high value does not mean the trait is fixed at birth or unchangeable; it means that, in this particular population and environment, genetic differences explain most of the observed spread.

Because heritability is a ratio bounded between 0 and 1 (or 0% and 100%), it is always specific to a population and an environment. A heritability of 0.5 for plant height in one field tells you nothing definitive about height heritability in a different climate or genetic background. Researchers, breeders, and students use a heritability calculator to standardize these estimates so they can compare traits, design breeding programs, and predict how a population will respond to selection.

Two distinct heritability statistics answer different practical questions, and this calculator reports both. Broad-sense heritability (H²) measures the proportion of phenotypic variance explained by all genetic effects combined — additive, dominance, and epistatic. It tells you how much of the trait spread is genetic in the broadest sense. Narrow-sense heritability (h²) isolates only the additive genetic variance, the component that parents reliably transmit to offspring.

This distinction matters enormously. Additive effects are the predictable, "heritable in the everyday sense" effects that respond to selection and pass cleanly from generation to generation. Dominance and epistatic effects depend on specific gene combinations that are reshuffled during meiosis, so they do not transmit reliably. For this reason, breeders care far more about narrow-sense heritability: it is h², not H², that predicts the response to artificial or natural selection.

Because additive variance is only part of the total genetic variance, narrow-sense heritability is always less than or equal to broad-sense heritability. The gap between them reflects the contribution of dominance variance (Vd) and epistasis. When H² greatly exceeds h², much of the genetic signal comes from non-additive interactions that selection cannot easily exploit.

The calculator partitions phenotypic variance and computes each heritability ratio exactly as shown below. First it sums the genetic and environmental variance to obtain the total phenotypic variance, then it divides the relevant genetic component by that total.

Broad-sense heritability uses the full genetic variance, while narrow-sense heritability uses only the additive variance. The leftover dominance variance is recovered as the difference between total genetic variance and additive variance, which is why a credible additive estimate must never exceed the total genetic variance.

The calculator also estimates realized heritability, an empirical value derived from an actual selection experiment rather than from a variance partition. This is the gold-standard validation of narrow-sense heritability because it is measured directly from how a population responds to selection over a generation.

Two quantities drive it. The selection differential (S) is how much the selected parents deviate from the original population mean — it captures the intensity of selection. The selection response (R) is how much the offspring mean shifts away from that same original population mean — it captures the actual genetic gain achieved. The calculator computes both from the population mean, parent mean, and offspring mean you enter.

Realized heritability is then the response per unit of selection differential, often written as the breeder's equation rearranged. If selecting heavier parents that average 10 units above the population yields offspring that average 5 units above the population, then half of the selection pressure was converted into permanent genetic gain, giving a realized heritability of 0.5. This single number summarizes how efficiently a trait responds to breeding.

A heritability value is a proportion, so reading it correctly is as important as computing it. The calculator reports H² and h² both as decimals and as percentages, alongside a variance breakdown that shows the genetic and environmental contributions to the total phenotypic variance.

• 0.0 to 0.2 (low): Most variation is environmental; the trait responds slowly to selection. Examples include many fitness-related and survival traits.
• 0.2 to 0.5 (moderate): A meaningful genetic component exists; breeding and selection can produce steady, predictable progress.
• 0.5 to 1.0 (high): Genetic differences dominate the observed spread; selection is highly effective. Skeletal and morphological traits often fall here.

Three cautions keep interpretation honest. First, heritability is population-specific: changing the environment or the gene pool changes the estimate. Second, a high heritability does not mean a trait is unchangeable — nutrition or treatment can shift the whole population mean without altering the heritability ratio. Third, heritability describes variation between individuals, not the cause of any single individual's trait value. Used with these caveats, this heritability calculator is a powerful tool for plant breeders, animal scientists, geneticists, and students learning quantitative genetics.