A cat has black fur. The allele for black fur (B) is dominant over the allele for white fur (b). The cat is heterozygous for this gene. Explain why this cat has black fur, using the terms genotype, phenotype, dominant and recessive in your answer.

Eduqas GCSE Biology — 7.2 Inheritance · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The cat’s genotype is Bb – it carries one dominant allele (B) and one recessive allele (b). The dominant allele for black fur is always expressed when present, so the B allele determines the observable trait. The recessive allele (b) for white fur is only expressed when no dominant allele is present. Therefore the cat’s phenotype is black fur because the dominant allele overrides the recessive one.

Examiner tips

  • Use the terms genotype, phenotype, dominant and recessive explicitly.
  • Show the logic: genotype → dominant allele expressed → phenotype.
  • Keep the answer concise – 4 marks only.
  • Mention that the recessive allele is silent in the presence of the dominant allele.

Common mistakes

  • Using the wrong order of alleles (e.g., bb instead of Bb).
  • Failing to explain that the dominant allele is expressed and the recessive is silent.
  • Not using the word ‘phenotype’ to describe the observable fur colour.

Mark scheme (4 marks)

  1. The cat's genotype is Bb (one dominant allele and one recessive allele)
  2. The dominant allele (B) for black fur is always expressed when present
  3. The recessive allele (b) for white fur is only expressed in the absence of the dominant allele
  4. Therefore the phenotype of the cat is black fur because the dominant allele determines the observable characteristic

Key terms in this question

genotype · phenotype · dominant · recessive · heterozygous

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