# Explain how epistasis can cause the phenotypic ratio observed in the F2 generation of a dihybrid cross to differ from the expected 9:3:3:1 ratio.

> IB DP Biology Higher Level (2023 syllabus) — D3.2 Inheritance · Explain · 4 marks

## Mark scheme (4 marks)

1. Epistasis occurs when one gene (the epistatic gene) masks or suppresses the expression of a different gene (the hypostatic gene)
2. This means two or more phenotypic classes are merged/combined, reducing the number of distinct phenotypes observed in the F2 generation
3. For example, in recessive epistasis the double-recessive homozygotes (aabb) mask expression of the B gene, so the 9:3:3:1 ratio becomes a 9:3:4 ratio
4. Because the two genes are non-allelic and interact, independent assortment alone is insufficient to predict phenotype frequencies; the biochemical/developmental pathway shared by both genes determines final phenotype

## Key terms

- [epistasis](https://www.gradenine.co.uk/glossary/epistasis)
- [dihybrid cross](https://www.gradenine.co.uk/glossary/dihybrid-cross)
- [F2 generation](https://www.gradenine.co.uk/glossary/f2-generation)
- [phenotypic ratio](https://www.gradenine.co.uk/glossary/phenotypic-ratio)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-epistasis-can-cause-the-505eb9d5) · Published by Druglandscape Ltd.