A dog breeder is studying coat colour in Labrador dogs. Black coat colour is controlled by a dominant allele (B). Yellow coat colour is caused by the recessive allele (b). The breeder crosses a homozygous black Labrador with a yellow Labrador. Explain what the genotype and phenotype of the offspring from this cross will be.

Edexcel GCSE Biology (1BI0) — 3.4 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 homozygous black parent has genotype BB and the yellow parent has genotype bb.
All offspring will have the genotype Bb (heterozygous).
All offspring will have a black coat (black phenotype).
Because B is dominant, the B allele is always expressed and the recessive b allele is masked in the presence of B.

Examiner tips

  • State the parental genotypes first. Show the Punnett square or explain the 1:1 genotype ratio. Mention that B is dominant and therefore the phenotype is black.
  • Use the exact terms "homozygous", "heterozygous", "dominant" and "recessive" as in the mark scheme.

Common mistakes

  • Confusing the parental genotypes (e.g., writing Bb × bb). Forgetting to state that the phenotype is black. Using vague terms like "black" without linking it to dominance.

Mark scheme (4 marks)

  1. The homozygous black parent has genotype BB and the yellow parent has genotype bb
  2. All offspring will have the genotype Bb / heterozygous
  3. All offspring will have a black coat / black phenotype
  4. Because B is dominant, the B allele is always expressed / the recessive b allele is masked in the presence of B

Key terms in this question

dominant · recessive · homozygous · genotype · phenotype

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