A farmer breeds guinea pigs. Some guinea pigs have smooth coat and some have rough coat. The allele for rough coat (R) is dominant over the allele for smooth coat (r). Two guinea pigs, both heterozygous for coat texture, are bred together. Explain why some of their offspring are likely to have smooth coat.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A farmer breeds guinea pigs. Some guinea pigs have smooth coat and some have rough coat. The allele for rough coat (R) is dominant over the allele for smooth coat (r). Two guinea pigs, both heterozygous for coat texture, are bred together.
Model answer (4 marks)
Both parents are heterozygous (Rr), so each carries one recessive allele (r). Each parent can pass on either the R or the r allele to offspring via gametes. Some offspring can inherit the r allele from both parents, giving the genotype rr. The smooth coat phenotype is recessive and is only expressed when two copies of the recessive allele are present (rr).
Examiner tips
- State that both parents are heterozygous (Rr).
- Explain that each parent can produce gametes with either allele.
- Show that offspring can receive r from both parents to form rr.
- Link the rr genotype to the smooth coat phenotype.
Common mistakes
- Failing to mention that both parents are heterozygous.
- Confusing dominant and recessive traits, e.g. saying smooth coat is dominant.
- Not explaining that rr is required for the smooth coat phenotype.
Mark scheme (4 marks)
- Both parents are heterozygous (Rr), so each parent carries one recessive allele (r)
- Each parent can pass on either the R or the r allele to offspring via gametes
- Some offspring can inherit the r allele from both parents, giving the genotype rr (homozygous recessive)
- The smooth coat phenotype is recessive and is only expressed when two copies of the recessive allele are present (rr)
Key terms in this question
Related
- Revision notes: Inheritance →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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