Explain why two pea plants with the same phenotype for seed texture can have different genotypes.

OCR GCSE Biology A: Gateway Science (J247) — B5.1 Inheritance · Explain · 4 marks · View as Markdown

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

Pea plants can produce either wrinkled seeds or round seeds. Round seed texture is controlled by a dominant allele (R) and wrinkled seed texture is controlled by a recessive allele (r). A scientist grows several pea plants and observes that some plants producing round seeds have different genotypes to one another.

Model answer (4 marks)

A round seed phenotype is produced when a plant carries at least one dominant R allele. A homozygous dominant plant (RR) has two R alleles and therefore shows the round seed phenotype. A heterozygous plant (Rr) also shows the round seed phenotype because the dominant R allele masks the recessive r allele. Thus two plants can both have round seeds but one can be RR and the other Rr, giving the same phenotype but different genotypes.

Examiner tips

Use the phrase ‘dominant allele is expressed when present’ to show understanding of dominance.
Explain both RR and Rr give round seeds, then state they differ in genotype.
Keep answer concise – 4 marks can be earned with 3–4 short sentences.

Common mistakes

Confusing phenotype with genotype – writing that both plants are RR.
Forgetting to mention the heterozygous Rr plant.
Using vague wording such as ‘different genetic makeup’ without specifying RR vs Rr.

Mark scheme (4 marks)

The dominant allele (R) is always expressed when present
A homozygous dominant plant (RR) has two dominant alleles and produces round seeds
A heterozygous plant (Rr) also produces round seeds because the recessive allele is not expressed
Therefore RR and Rr have the same phenotype (round seeds) but different genotypes

Key terms in this question

phenotype · genotype