A dog breeder is investigating coat length in dogs. The allele for long coat (L) is dominant over the allele for short coat (l). A heterozygous long-coated dog is crossed with a homozygous short-coated dog. Describe the possible genotypes and phenotypes of the offspring from this cross, and explain why some offspring may have a short coat despite one parent having a long coat.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Parental genotypes: Ll (heterozygous long coat) × ll (homozygous short coat)
Offspring genotypes: 50 % Ll, 50 % ll (1:1 ratio)
Offspring phenotypes: 50 % long coat (Ll) and 50 % short coat (ll)
Explanation: The short‑coat allele l is recessive; it is only expressed when an individual has two copies (ll). The heterozygous parent can pass on either L or l, so some offspring receive the l allele from both parents and show a short coat.
Offspring genotypes: 50 % Ll, 50 % ll (1:1 ratio)
Offspring phenotypes: 50 % long coat (Ll) and 50 % short coat (ll)
Explanation: The short‑coat allele l is recessive; it is only expressed when an individual has two copies (ll). The heterozygous parent can pass on either L or l, so some offspring receive the l allele from both parents and show a short coat.
Examiner tips
- State both parental genotypes first
- Show the 1:1 genotype ratio and link to phenotypes
- Explain recessive nature of l and why it can appear in offspring
Common mistakes
- Confusing the parental genotypes (e.g., writing ll × ll)
- Failing to mention the 1:1 ratio
- Not explaining that l is recessive and requires two copies
Mark scheme (4 marks)
- Correct identification of the two parental genotypes used in the cross (Ll × ll)
- Correct offspring genotypes stated as Ll and ll (50% each / 1:1 ratio)
- Correct offspring phenotypes stated: 50% long coat and 50% short coat
- Explanation that the short-coat allele (l) is recessive and is only expressed when two copies are present (homozygous recessive genotype ll), which is possible because the heterozygous parent can pass on the l allele
Key terms in this question
heterozygous · homozygous · dominant · genotype · phenotype · allele
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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