# 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

> 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.

## 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

- [allele](https://www.gradenine.co.uk/glossary/allele)
- [genotype](https://www.gradenine.co.uk/glossary/genotype)
- [phenotype](https://www.gradenine.co.uk/glossary/phenotype)
- [heterozygous](https://www.gradenine.co.uk/glossary/heterozygous)
- [dominant](https://www.gradenine.co.uk/glossary/dominant)
- [recessive](https://www.gradenine.co.uk/glossary/recessive)

## Related

- [Revision notes for OCR A-Level Biology B — Advancing Biology (H422)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-gardener-is-growing-tomato-plants-b37bbb00) · Published by Druglandscape Ltd.