# A breed of cattle can display one of two coat colours: red or white. When a red bull is crossed with a white cow, all offspring have a roan coat, which appears as a mixture of red and white hairs. Explain why the roan offspring, when crossed with each other, produce red, roan, and white calves in an expected ratio of 1:2:1.

> OCR A-Level Biology A (H420) — 6.2 Patterns of inheritance · Explain · 4 marks

> In this breed of cattle, coat colour is controlled by a single gene with two alleles. Neither allele is dominant over the other.

## Mark scheme (4 marks)

1. Identifies the inheritance pattern as codominance
2. States that roan individuals are heterozygous, carrying one allele for red and one for white
3. States or implies that crossing two heterozygous roan parents produces all three possible genotype combinations
4. Links genotype to phenotype to explain the 1:2:1 ratio — homozygous red is red, heterozygous is roan (both alleles expressed), homozygous white is white

## Key terms

- [roan](https://www.gradenine.co.uk/glossary/roan)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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/a-breed-of-cattle-can-display-3b6ac8d1) · Published by Druglandscape Ltd.