In cats, coat colour is controlled by a single gene. The allele for black coat (B) is dominant over the allele for ginger coat (b). A female cat with a black coat is crossed with a male cat with a ginger coat. Half of the offspring have black coats and half have ginger coats. Explain what these results tell us about the genotype of the female parent cat, and state the genotypes of all the offspring produced.

Pearson Edexcel International GCSE Biology (4BI1) — 3.2 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 female cat must be heterozygous (Bb). A homozygous dominant (BB) female crossed with a ginger (bb) male would give all black offspring, so the 50:50 ratio shows heterozygosity.

Offspring genotypes:
• Black‑coat cats: Bb
• Ginger‑coat cats: bb

Examiner tips

  • State the female’s genotype first, then explain why a 50:50 ratio rules out BB. Use the Punnett square logic to list offspring genotypes. Mention that black phenotype comes from Bb, ginger from bb.

Common mistakes

  • Assuming the female is BB and giving all black offspring. Failing to link the 50:50 ratio to heterozygosity. Listing only phenotypes without specifying genotypes.

Mark scheme (4 marks)

  1. The female parent cat must be heterozygous (Bb)
  2. Because a homozygous dominant (BB) female crossed with a ginger (bb) male would produce all black offspring, not a 50:50 ratio
  3. Black-coated offspring have genotype Bb
  4. Ginger-coated offspring have genotype bb

Key terms in this question

allele · dominant · genotype

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