# Polygenic trait

A trait controlled by many genes, often plus the environment. These traits vary continuously, giving a bell-shaped spread rather than clear categories.

Biology (university) · Part 07: Genetics and inheritance · Card 8 of 8 · Last updated 2026-10-19

## Analogy

A mixing desk with many sliders, not one switch.

## In practice

Human height and skin colour. Genes set the range and nutrition affects where within it a person ends up.

## Practice

### Flashcards

- **What kind of variation does a polygenic trait show?** Continuous variation, a smooth bell-shaped spread.

## Questions people ask

### Why are traits like height not simple 3:1 ratios?

Height depends on many genes, each adding a small effect, and on the environment, such as nutrition. With many gene combinations possible, people fall along a smooth range rather than in clear groups. Plotting heights in a large group gives a bell-shaped curve.

### Does the environment affect gene expression?

Yes. Genes set a range of possibilities, and the environment influences where within it an individual ends up. A child with genes for tall height may not reach it with poor nutrition. Identical twins raised apart can differ in weight, health and skills because of their different environments.

## Related concepts

- [Allele](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-1)
- [Law of segregation](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-2)
- [Law of independent assortment](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-3)
- [Incomplete dominance and codominance](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-4)

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