In rabbits, fur colour is controlled by a single gene. The allele for brown fur (B) is dominant over the allele for white fur (b). Two rabbits that are both heterozygous for fur colour are bred together. Describe the expected genotypes and phenotypes of the offspring produced, and explain why some offspring may have white fur even though neither parent has white fur.

Pearson Edexcel International GCSE Biology (4BI1) — 3.2 Inheritance · Describe and Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Both parents are heterozygous (Bb). The Punnett square gives offspring genotypes BB, Bb and bb in a 1:2:1 ratio. BB and Bb offspring have brown fur, so three‑quarters of the litter are brown. The bb offspring are white; each parent carried a recessive b allele that was not expressed because it was masked by the dominant B allele.

Examiner tips

  • State the parental genotypes first; then give the Punnett square result and the 1:2:1 ratio. Mention the phenotype for each genotype. Explain that the white offspring are bb and that the recessive allele was hidden in the parents.
  • Use the exact terms ‘heterozygous’, ‘homozygous recessive’, ‘dominant’, and ‘recessive’.
  • Show the ratio 1:2:1 and the 75% brown, 25% white outcome.

Mark scheme (4 marks)

  1. Both parents have the genotype Bb (heterozygous)
  2. Offspring genotypes are BB, Bb and bb (in a 1:2:1 ratio) / three possible genotypes identified correctly
  3. Offspring with genotype BB or Bb have brown fur / three out of four offspring are expected to have brown fur
  4. White fur offspring are homozygous recessive (bb) / each parent carried a recessive b allele that was not expressed / the b allele is masked by the dominant B allele in the parents

Key terms in this question

allele · dominant · heterozygous · genotype · phenotype

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