A dog breeder is investigating coat length in Labrador retrievers. Coat length is controlled by a single gene. The allele for short coat (S) is dominant and the allele for long coat (s) is recessive. Two dogs that are both heterozygous for coat length are bred together. Explain why some of their offspring will have a long coat, using a Punnett square in your answer.

OCR A-Level Biology B — Advancing Biology (H422) — 5.2 Inheritance and variation · Explain · 5 marks · View as Markdown

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

In Labrador retrievers, short coat (S) is dominant over long coat (s). Two heterozygous short-coated dogs (Ss) are crossed.

Model answer (5 marks)

Both parents are heterozygous (Ss).

Punnett square:

  | S | s
---|----|----
S | SS | Ss
s | Ss | ss

The ss genotype contains only recessive alleles, so the long‑coat phenotype is expressed. One quarter (25 %) of the offspring are expected to be long‑coated.

Examiner tips

  • State both parents are Ss first. Show the full Punnett square. Explain that ss gives long coat. Give the 25 % figure.

Common mistakes

  • Omitting the Punnett square or showing it incorrectly. Failing to explain that ss produces the long‑coat phenotype.

Mark scheme (5 marks)

  1. Both parents are heterozygous, so their genotype is Ss
  2. Correct Punnett square showing all four possible offspring genotypes: SS, Ss, Ss, ss
  3. Some offspring inherit the recessive allele (s) from both parents, giving genotype ss
  4. The ss genotype means only recessive alleles are present, so the long coat phenotype is expressed
  5. One quarter (25%) of the offspring are expected to have the long coat phenotype

Key terms in this question

allele · heterozygous · dominant · recessive · Punnett square

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