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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
Both parents are heterozygous (Bb), so each can give either B or b.
Each offspring receives one allele from each parent, so the possible genotypes are BB, Bb or bb.
Only when an offspring receives a recessive allele from both parents (bb) will the recessive trait be expressed.
Because the recessive allele is masked by the dominant allele, only the bb genotype shows white fur.
Thus some offspring are white because they inherit b from both parents, giving the genotype bb.
Each offspring receives one allele from each parent, so the possible genotypes are BB, Bb or bb.
Only when an offspring receives a recessive allele from both parents (bb) will the recessive trait be expressed.
Because the recessive allele is masked by the dominant allele, only the bb genotype shows white fur.
Thus some offspring are white because they inherit b from both parents, giving the genotype bb.
Examiner tips
- Show the Punnett square logic – list all allele combinations. Explain why bb is the only genotype that expresses white. Use the terms heterozygous, homozygous recessive, dominant and recessive. Keep the answer concise and directly tied to the mark scheme points.
Common mistakes
- Saying both parents are homozygous instead of heterozygous. Claiming white fur appears in Bb individuals. Using vague terms like ‘chance’ without linking to recessive allele inheritance.
Mark scheme (5 marks)
- Both parents are heterozygous (Bb), meaning each parent carries one dominant and one recessive allele
- Each parent passes one allele to each offspring via gametes / gametes are haploid and carry only one allele
- There is a chance that an offspring inherits a recessive allele (b) from each parent, giving the genotype bb
- The recessive allele (b) is only expressed when two copies are present / no dominant allele is present to mask it
- White-furred offspring have the genotype bb / are homozygous recessive
Key terms in this question
allele · dominant · heterozygous · genotype
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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