A gardener is growing tomato plants. The gene for fruit colour in tomatoes has two alleles: red fruit (R) is dominant and yellow fruit (r) is recessive. Two tomato plants, both with red fruit, are crossed. The offspring produced include both red-fruited and yellow-fruited plants. Explain how two red-fruited parent plants can produce yellow-fruited offspring, using the terms genotype, phenotype, heterozygous, dominant and recessive in your answer.

OCR A-Level Biology B — Advancing Biology (H422) — 5.2 Inheritance and variation · Explain · 5 marks · View as Markdown

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

A gardener notices that when two red-fruited tomato plants are bred together, some of the offspring produce yellow fruit. The allele for red fruit (R) is dominant and the allele for yellow fruit (r) is recessive.

Model answer (5 marks)

Both parent plants are heterozygous (Rr). Each plant carries one dominant allele (R) and one recessive allele (r). The red phenotype is expressed because the dominant allele is present, so only one copy of R is required for red fruit. During fertilisation each parent contributes one allele to the zygote. If both parents pass the recessive allele (r) to the offspring, the resulting genotype is rr. This homozygous recessive genotype shows the yellow phenotype because the recessive allele is only expressed when no dominant allele is present.

Examiner tips

  • Use the exact terms: genotype, phenotype, heterozygous, dominant, recessive
  • Show the logic of dominance – one R gives red, rr gives yellow
  • Explain gamete inheritance – each parent can give r
  • Keep answer concise – 5 marks, no extra words

Common mistakes

  • Assuming both parents are homozygous RR
  • Confusing phenotype with genotype
  • Not explaining why rr gives yellow

Mark scheme (5 marks)

  1. Both parent plants must be heterozygous (Rr), meaning they carry one dominant allele and one recessive allele
  2. The red phenotype is shown because R is dominant, so only one copy of R is needed for red fruit to be expressed
  3. Each parent passes one allele to the offspring via gametes, so offspring can inherit an r allele from each parent
  4. Yellow-fruited offspring have the genotype rr (homozygous recessive), meaning two copies of the recessive allele are present
  5. The recessive allele (r) is only expressed in the phenotype when no dominant allele is present, i.e. when the genotype is rr

Key terms in this question

allele · genotype · phenotype · heterozygous · dominant · recessive

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