# A dog breeder is selecting dogs with different coat colours for a breeding programme. In Labrador dogs, coat colour is controlled by a single gene. The allele for black coat (B) is dominant over the allele for chocolate coat (b). Two black Labradors are crossed and produce a litter that includes both black and chocolate puppies. Explain how two black Labradors can produce chocolate-coloured offspring.

> OCR GCSE Biology A: Gateway Science (J247) — B5.1 Inheritance · Explain · 4 marks

> In Labrador dogs, coat colour is controlled by a single gene. The allele for black coat (B) is dominant, and the allele for chocolate coat (b) is recessive.

## Mark scheme (4 marks)

1. The chocolate coat allele (b) is recessive, so it is only expressed when two copies are present (homozygous recessive, bb).
2. Both black parent dogs must be heterozygous (Bb), carrying one dominant black allele and one recessive chocolate allele.
3. Each parent passes on one allele to offspring via gametes, and a puppy can inherit the recessive allele (b) from each parent.
4. If a puppy inherits the recessive allele from both parents (genotype bb), it will have the chocolate coat phenotype.

## Key terms

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

## Related

- [Revision notes for OCR GCSE Biology A: Gateway Science (J247)](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-selecting-dogs-e641c84f) · Published by Druglandscape Ltd.