A gardener notices that some pea plants in their garden produce round seeds and others produce wrinkled seeds. Round seeds are caused by a dominant allele (R) and wrinkled seeds are caused by a recessive allele (r). A pea plant that is homozygous dominant is crossed with a pea plant that is homozygous recessive. Explain what is meant by the terms dominant and recessive, and describe the genotype and phenotype of the offspring produced from this cross.

Edexcel GCSE Biology (1BI0) — 3.4 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)

Dominant means an allele that is always expressed in the phenotype, even when only one copy is present.
Recessive means an allele that is only expressed when no dominant allele is present, i.e. when two copies are present.
The cross is between RR (homozygous dominant) and rr (homozygous recessive). All offspring receive one R allele from the RR parent and one r allele from the rr parent, giving the genotype Rr.
Because the R allele is dominant, the phenotype of all offspring is round seeds.

Examiner tips

  • Use the words ‘dominant’ and ‘recessive’ with their definitions; include ‘expressed’ and ‘absence of a dominant allele’.
  • State the exact genotype (Rr) and phenotype (round) for all offspring.

Mark scheme (4 marks)

  1. Dominant means an allele that is always expressed (in the phenotype) / expressed when only one copy is present
  2. Recessive means an allele that is only expressed in the absence of a dominant allele / when two copies are present
  3. All offspring have the genotype Rr (heterozygous)
  4. All offspring have round seeds as their phenotype / the dominant phenotype is expressed in all offspring

Key terms in this question

dominant · recessive · genotype · phenotype · homozygous

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