In pea plants, seed shape is controlled by a single gene. The allele for round seeds (R) is dominant over the allele for wrinkled seeds (r). Two pea plants that both have round seeds are crossed. Some of their offspring have wrinkled seeds. Explain how this is possible.

WJEC GCSE Biology (Wales) — 2.3.2 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 round but carry the recessive r allele (Rr). R is dominant, so they show round seeds. During gamete formation the alleles separate, each parent passes one allele to the offspring. An offspring that receives r from both parents (rr) will have wrinkled seeds.

Examiner tips

  • Use the term ‘heterozygous’ for the parents (Rr).
  • Explain dominance and segregation of alleles.
  • Show that rr gives wrinkled phenotype.
  • Keep answer concise and use correct terminology.

Common mistakes

  • Saying parents are homozygous dominant (RR).
  • Forgetting to mention that R is dominant over r.
  • Not linking the rr genotype to the wrinkled phenotype.

Mark scheme (4 marks)

  1. Both parent plants must carry the recessive allele for wrinkled seeds
  2. The parents do not show wrinkled seeds because R is dominant over r
  3. Each parent passes one allele to the offspring / alleles separate during gamete formation
  4. An offspring that inherits the recessive allele from both parents (rr) will have wrinkled seeds

Key terms in this question

allele · dominant

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