# A dog breeder is investigating coat length in Labrador retrievers. Coat length is controlled by a single gene. The allele for short coat (S) is dominant and the allele for long coat (s) is recessive. Two dogs that are both heterozygous for coat length are bred together. Explain why some of their offspring will have a long coat, using a Punnett square in your answer.

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

> In Labrador retrievers, short coat (S) is dominant over long coat (s). Two heterozygous short-coated dogs (Ss) are crossed.

## Mark scheme (5 marks)

1. Both parents are heterozygous, so their genotype is Ss
2. Correct Punnett square showing all four possible offspring genotypes: SS, Ss, Ss, ss
3. Some offspring inherit the recessive allele (s) from both parents, giving genotype ss
4. The ss genotype means only recessive alleles are present, so the long coat phenotype is expressed
5. One quarter (25%) of the offspring are expected to have the long coat phenotype

## Key terms

- [allele](https://www.gradenine.co.uk/glossary/allele)
- [heterozygous](https://www.gradenine.co.uk/glossary/heterozygous)
- [dominant](https://www.gradenine.co.uk/glossary/dominant)
- [recessive](https://www.gradenine.co.uk/glossary/recessive)
- [Punnett square](https://www.gradenine.co.uk/glossary/punnett-square)

## 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-dog-breeder-is-investigating-coat-4d030974) · Published by Druglandscape Ltd.