# Explain how a dihybrid cross between two heterozygous parents for seed shape (round, R, dominant over wrinkled, r) and seed colour (yellow, Y, dominant over green, y) produces offspring in a 9:3:3:1 phenotypic ratio, assuming the two genes are on different chromosomes.

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

## Mark scheme (4 marks)

1. Each parent is heterozygous for both genes (RrYy), producing four types of gametes (RY, Ry, rY, ry) in equal proportions.
2. Because the genes are on different (non-homologous) chromosomes, they assort independently during meiosis (independent assortment / Mendel's Law of Independent Assortment).
3. Random fusion of the four male and four female gametes produces 16 equally probable genotypic combinations, giving 9 offspring with at least one dominant allele at both loci (round and yellow), 3 with dominant allele only at the R locus (round and green), 3 with dominant allele only at the Y locus (wrinkled and yellow), and 1 homozygous recessive at both loci (wrinkled and green).
4. The 9:3:3:1 ratio arises because each trait individually gives a 3:1 ratio (due to dominance), and the two independent 3:1 ratios combine multiplicatively (3×3 : 3×1 : 1×3 : 1×1).

## Key terms

- [dihybrid cross](https://www.gradenine.co.uk/glossary/dihybrid-cross)
- [heterozygous](https://www.gradenine.co.uk/glossary/heterozygous)
- [phenotypic ratio](https://www.gradenine.co.uk/glossary/phenotypic-ratio)

## Related

- [Revision notes for IB DP Biology Standard 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-a-dihybrid-cross-between-9147d414) · Published by Druglandscape Ltd.