In guinea pigs, coat colour is controlled by a single gene. The allele for black coat (B) is dominant over the allele for white coat (b). Two black-coated guinea pigs are bred together and some of their offspring have white coats. Explain how this is possible.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Both parents are black, but each carries one recessive allele (Bb). Each parent can pass either B or b to an offspring. An offspring receives one allele from each parent, so a bb genotype can occur. A bb genotype shows a white coat because there is no dominant B to mask it.
Examiner tips
- State that both parents are heterozygous (Bb)
- Explain that each parent can pass either allele
- Show that offspring inherit one allele from each parent
- Link bb genotype to white phenotype
Common mistakes
- Assuming parents are homozygous BB
- Failing to mention that offspring inherit one allele from each parent
- Confusing genotype with phenotype
Mark scheme (4 marks)
- Both parents must carry the recessive allele for white coat
- Each parent can pass on either allele to their offspring
- An offspring inherits one allele from each parent
- An offspring with the genotype bb shows the white coat phenotype because there is no dominant allele to mask it
Key terms in this question
Related
- All WJEC GCSE Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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