A dog breeder is selecting dogs with different coat colours for a breeding programme. In Labrador dogs, coat colour is controlled by a single gene. The allele for black coat (B) is dominant over the allele for chocolate coat (b). Two black Labradors are crossed and produce a litter that includes both black and chocolate puppies. Explain how two black Labradors can produce chocolate-coloured offspring.

OCR GCSE Biology A: Gateway Science (J247) — B5.1 Inheritance · Explain · 4 marks · View as Markdown

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

In Labrador dogs, coat colour is controlled by a single gene. The allele for black coat (B) is dominant, and the allele for chocolate coat (b) is recessive.

Model answer (4 marks)

Both black parents must be heterozygous (Bb). Each passes one allele to the offspring. If a puppy receives the recessive allele (b) from both parents, its genotype is bb, so the recessive chocolate coat is expressed.

Examiner tips

  • State that black dogs can be Bb, not BB
  • Explain that chocolate appears only when both parents contribute b
  • Show the inheritance of one allele from each parent

Common mistakes

  • Assuming both parents are BB
  • Claiming chocolate appears with one b allele
  • Not linking genotype bb to chocolate phenotype

Mark scheme (4 marks)

  1. The chocolate coat allele (b) is recessive, so it is only expressed when two copies are present (homozygous recessive, bb).
  2. Both black parent dogs must be heterozygous (Bb), carrying one dominant black allele and one recessive chocolate allele.
  3. Each parent passes on one allele to offspring via gametes, and a puppy can inherit the recessive allele (b) from each parent.
  4. If a puppy inherits the recessive allele from both parents (genotype bb), it will have the chocolate coat phenotype.

Key terms in this question

allele · dominant

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