# A genetics student is investigating fur colour in rabbits. Fur colour is controlled by a single gene with two alleles. The allele for brown fur (B) is dominant over the allele for white fur (b). Two heterozygous brown rabbits are crossed. Explain why some of the offspring from this cross are expected to have white fur, and describe the genotypes of the white-furred offspring.

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

> In rabbits, fur colour shows discontinuous variation. The allele for brown fur (B) is dominant and the allele for white fur (b) is recessive. Both parent rabbits have brown fur but carry the recessive allele.

## Mark scheme (5 marks)

1. Both parents are heterozygous (Bb), meaning each parent carries one dominant and one recessive allele
2. Each parent passes one allele to each offspring via gametes / gametes are haploid and carry only one allele
3. There is a chance that an offspring inherits a recessive allele (b) from each parent, giving the genotype bb
4. The recessive allele (b) is only expressed when two copies are present / no dominant allele is present to mask it
5. White-furred offspring have the genotype bb / are homozygous recessive

## Key terms

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

## 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-genetics-student-is-investigating-fur-92b54b68) · Published by Druglandscape Ltd.