# A farmer notices that some of her sheep have black wool and some have white wool. Wool colour is controlled by a single gene. The allele for white wool (W) is dominant over the allele for black wool (w). The farmer crosses a heterozygous white-wool sheep with a black-wool sheep. Explain the possible genotypes and phenotypes of the offspring from this cross, and predict the proportion of offspring expected to show each phenotype.

> OCR A-Level Biology B — Advancing Biology (H422) — 5.2 Inheritance and variation · Explain · 5 marks

> Wool colour in sheep shows discontinuous variation. The white wool allele (W) is dominant and the black wool allele (w) is recessive. A heterozygous white-wool sheep is crossed with a black-wool sheep.

## Mark scheme (5 marks)

1. Correctly identifies the genotype of the heterozygous white-wool parent as Ww
2. Correctly identifies the genotype of the black-wool parent as ww
3. States the two possible offspring genotypes as Ww and ww
4. States that Ww offspring will have a white phenotype because W is dominant / only one dominant allele is needed for white wool to be expressed
5. Predicts a 1:1 ratio / 50% white-wool offspring (Ww) to 50% black-wool offspring (ww)

## Key terms

- [allele](https://www.gradenine.co.uk/glossary/allele)
- [dominant](https://www.gradenine.co.uk/glossary/dominant)
- [genotype](https://www.gradenine.co.uk/glossary/genotype)
- [phenotype](https://www.gradenine.co.uk/glossary/phenotype)
- [heterozygous](https://www.gradenine.co.uk/glossary/heterozygous)

## Related

- [Revision notes for OCR A-Level Biology B — Advancing Biology (H422)](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-farmer-notices-that-some-of-b90a5d3b) · Published by Druglandscape Ltd.