--- title: "Quantitative Traits, Variance Components and Heritability" description: "Many quantitative traits reflect several genes and environmental effects. Under stated assumptions, phenotypic variance can be divided into components. Broad-sense heritability uses total genetic vari" canonical: https://lightmysky.com/learn/science/quantitative-traits-variance-components-and-heritability-mt_orJhIlYnKM source: https://lightmysky.com/learn/science/quantitative-traits-variance-components-and-heritability-mt_orJhIlYnKM.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Quantitative Traits, Variance Components and Heritability Many quantitative traits reflect several genes and environmental effects. Under stated assumptions, phenotypic variance can be divided into components. Broad-sense heritability uses total genetic variance; narrow-sense heritability uses additive genetic variance. Both describe a specified population and environment, not an individual's inherited percentage. Subject: Science · Area: Genetics & Evolution · Ages 18 to 20 Page: https://lightmysky.com/learn/science/quantitative-traits-variance-components-and-heritability-mt_orJhIlYnKM ## Ready when they can - Splits stated variance components under a simplified model and calculates broad-sense or narrow-sense heritability. - Predicts how heritability changes when environmental variance changes, with the relevant genetic variance and model assumptions held fixed. - Rejects the interpretation that a heritability of 0.6 means sixty per cent of an individual's trait comes from genes. ## Lesson: Why continuous traits need variance Quantitative traits, such as height, are measured on a numerical scale. Many reflect several genes and environmental effects. They can vary continuously, but their distribution need not be an exact bell curve. The mean describes the centre of the measurements. Variance measures their spread around that mean. A difference between two means does not tell you their variances. Use the simplified model V_P = V_G + V_E: phenotypic variance equals genetic plus environmental variance. Here we assume no genotype-environment covariance and omit genotype-by-environment interaction. Additive genetic variance V_A describes variation associated with average allele effects that add together. Other genetic components can include dominance and interactions between genes, like the epistasis studied earlier. *(drawing: In this example, genetic variance is 30 + 12 + 8 = 50. Environmental variance is 50, so phenotypic variance is 100.)* **Example.** Broad-sense heritability H² = V_G/V_P divides total genetic variance by phenotypic variance. Narrow-sense h² = V_A/V_P uses additive genetic variance in the numerator. The superscript is part of the notation; do not square the ratio again. Here V_P = 100, V_G = 50 and V_A = 30, so H² = 0.5 and h² = 0.3. Use h² in the basic prediction of response to selection. Heritability depends on the population and conditions. With V_P = V_G + V_E and V_G fixed, decreasing V_E raises H²; increasing V_E lowers it. The same reasoning applies to h² if V_A is also fixed. More uniform conditions may reduce V_E, but environmental changes can also alter genetic effects. Unchanged alleles alone do not fix every variance component. *(drawing: With genetic variance fixed at 40, H² is 40/60 in setting A and 40/120 in setting B.)* H² = 0.6 assigns 60 percent of phenotypic variance to genetic differences in the specified population and conditions. It does not divide one person's height into genetic and environmental percentages. In an experiment with matching genetic distributions, different nutrient treatments can change group mean heights. That environmental effect on a mean does not give a heritability estimate. **Recap.** H² uses genetic variance; h² uses additive genetic variance. Both describe variation in a specified population and environment, not percentages of one person's trait. *Lesson, questions and flashcards written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Epistasis and Reading Pathway Order from Double Mutants](https://lightmysky.com/learn/science/epistasis-and-reading-pathway-order-from-double-mutants-mt_eE2WRPyYPk) ## Opens up - [Association Studies and What They Can and Cannot Show](https://lightmysky.com/learn/science/association-studies-and-what-they-can-and-cannot-show-mt_IFFUFX3xrv) - [Selection on Quantitative Traits and Predicting the Response](https://lightmysky.com/learn/science/selection-on-quantitative-traits-and-predicting-the-response-mt_M-czTLDmfl)