In rabbits, the allele for dark fur (D) is dominant over the allele for white fur (d). A rabbit breeder suspects that a dark-furred rabbit may be heterozygous. Describe how a test cross could be carried out to determine the genotype of this dark-furred rabbit, and explain how the results would allow the breeder to identify whether the rabbit is homozygous dominant or heterozygous.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Cross the dark‑furred rabbit (unknown genotype) with a white‑furred rabbit that is homozygous recessive (dd).
1. Mate the two rabbits and allow them to produce a large number of offspring.
2. Record the fur colour of each offspring.
3. If all offspring are dark‑furred, the unknown rabbit must be homozygous dominant (DD).
4. If about half of the offspring are white‑furred, the unknown rabbit is heterozygous (Dd).
1. Mate the two rabbits and allow them to produce a large number of offspring.
2. Record the fur colour of each offspring.
3. If all offspring are dark‑furred, the unknown rabbit must be homozygous dominant (DD).
4. If about half of the offspring are white‑furred, the unknown rabbit is heterozygous (Dd).
Examiner tips
- Use the exact terms DD, Dd, dd and explain the phenotypic ratios.
- Mention that a large sample size increases reliability.
- Show the logic: all dark → DD, ~50% white → Dd.
Common mistakes
- Failing to state that the test cross uses a homozygous recessive (dd) partner.
- Confusing the phenotypic ratio with the genotype of the test cross parent.
- Not recognising that a small sample size can give misleading results.
Mark scheme (4 marks)
- The dark-furred rabbit must be crossed with a homozygous recessive (white-furred) rabbit.
- If all offspring show the dominant phenotype (dark fur), the original rabbit is homozygous dominant (DD).
- If approximately half the offspring show the recessive phenotype (white fur), the original rabbit is heterozygous (Dd).
- A large number of offspring should be produced / the more offspring, the more reliable the result.
Key terms in this question
test cross · homozygous · heterozygous · dominant · genotype
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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