In cats, the allele for black fur (B) is dominant over the allele for ginger fur (b). A breeder wishes to find out whether a black cat is homozygous dominant or heterozygous. Describe how a test cross could be carried out and explain how the offspring would indicate the genotype of the black cat.

Cambridge International IGCSE Biology (0610) — 17.4 Monohybrid inheritance · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Cross the black cat with a homozygous recessive ginger cat (bb).

If the black cat is BB, all offspring will be Bb and show black fur.
If the black cat is Bb, the offspring will be 50 % Bb (black) and 50 % bb (ginger).

Examiner tips

  • State the cross clearly (black × bb).
  • Explain the expected phenotypes for BB × bb and Bb × bb.
  • Use correct genetic symbols (BB, Bb, bb).
  • Show the link between offspring phenotypes and the black cat’s genotype.

Common mistakes

  • Using the wrong recessive parent (e.g., crossing with a black cat).
  • Failing to mention that all black offspring indicate BB.

Mark scheme (4 marks)

  1. The black cat must be crossed with a homozygous recessive (ginger) cat
  2. If all offspring show the dominant phenotype (black fur), the black cat is homozygous dominant (BB)
  3. If approximately half the offspring show the recessive phenotype (ginger fur), the black cat is heterozygous (Bb)
  4. Correct use of genetic notation — parent genotypes correctly stated as BB or Bb for the black cat and bb for the ginger cat, with offspring genotypes consistent with the cross described

Key terms in this question

test cross · homozygous · heterozygous · dominant · genotype

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