In pea plants, the allele for round seeds (R) is dominant over the allele for wrinkled seeds (r). A homozygous round-seeded plant is crossed with a homozygous wrinkled-seeded plant. Describe the genotypes and phenotypes of the offspring produced from this cross, and explain why all offspring show the same phenotype.

Eduqas GCSE Biology — 7.2 Inheritance · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

All offspring are heterozygous Rr.
All offspring show a round‑seeded phenotype.
The R allele is dominant and is expressed whenever it is present.
The recessive r allele is not expressed because there is no second r allele (or because a dominant allele is present).

Examiner tips

  • Use the exact terms ‘heterozygous Rr’ and ‘round‑seeded phenotype’.
  • Explain dominance by stating the R allele is expressed and the r allele is not when only one copy is present.
  • Show the logic that all gametes carry R from the round parent and r from the wrinkled parent, giving only Rr zygotes.

Common mistakes

  • Writing the genotype as RR or rr instead of Rr.
  • Claiming both alleles are expressed or that the phenotype is mixed.
  • Using informal language such as ‘dominant means it shows up’ instead of ‘R is dominant and is expressed’.

Mark scheme (4 marks)

  1. The genotype of all offspring is Rr (heterozygous)
  2. All offspring have a round-seeded phenotype
  3. The R allele is dominant and is always expressed when present
  4. The recessive allele (r) is not expressed because there is no second recessive allele present / a dominant allele is present

Key terms in this question

dominant · genotype · phenotype · homozygous

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