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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Epistasis is a genetic interaction in which one gene (the epistatic gene) masks the expression of another gene (the hypostatic gene). In a dihybrid cross, this masking merges two phenotypic classes, so the number of distinct phenotypes in the F2 is reduced. For example, in recessive epistasis the aabb genotype blocks the effect of the B gene; the expected 9:3:3:1 ratio becomes 9:3:4 because the two recessive classes combine. Since the genes are non‑allelic, independent assortment alone cannot predict the phenotype frequencies; the biochemical pathway shared by the two genes determines the final phenotype.
Examiner tips
- Use the term ‘epistasis’ and explain it as masking of one gene by another.
- Show how the merging of two phenotypic classes changes the ratio (e.g. 9:3:3:1 to 9:3:4).
- Mention that the genes are non‑allelic and that the pathway interaction matters.
Common mistakes
- Confusing epistasis with dominance or incomplete dominance.
- Failing to specify that the two genes are non‑allelic.
- Giving the wrong modified ratio (e.g. 9:3:2:1 instead of 9:3:4).
Mark scheme (4 marks)
- Epistasis occurs when one gene (the epistatic gene) masks or suppresses the expression of a different gene (the hypostatic gene)
- This means two or more phenotypic classes are merged/combined, reducing the number of distinct phenotypes observed in the F2 generation
- 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
- 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 in this question
epistasis · dihybrid cross · F2 generation · phenotypic ratio
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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