In guinea pigs, the allele for short hair (S) is dominant over the allele for long hair (s). A breeder crosses two guinea pigs, both with short hair, and obtains offspring in an approximate ratio of 3 short-haired : 1 long-haired. Explain how this ratio arises, including the genotypes of the parents and the possible genotypes of the offspring.

Cambridge International IGCSE Biology (0610) — 17.4 Monohybrid 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)

Both parents are heterozygous (Ss). Each parent can produce two types of gamete: S or s. The possible offspring genotypes are SS, Ss, Ss and ss. The allele S is dominant, so SS and Ss give the short‑hair phenotype, while only ss gives the long‑hair phenotype, producing a 3:1 ratio.

Examiner tips

  • State the parents’ genotypes first; this shows you understand the cross.
  • Show the Punnett square or list the gametes to justify the 3:1 ratio.
  • Explain the dominance of S to link genotype to phenotype.

Common mistakes

  • Assuming the parents are SS, giving a 1:0 ratio.
  • Forgetting that both Ss and SS give short hair, so only ss is long-haired.
  • Not mentioning the dominance of S, so the ratio explanation is incomplete.

Mark scheme (4 marks)

  1. Both parents are heterozygous (Ss)
  2. Each parent produces two types of gamete: S and s
  3. The possible offspring genotypes are SS, Ss, Ss and ss (three short-haired : one long-haired)
  4. S is dominant, so SS and Ss both show the short-hair phenotype; only ss shows the long-hair phenotype, producing the 3:1 ratio

Key terms in this question

dominant · genotype

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