# A student crosses two true-breeding pea plants. One parent has purple flowers and the other has white flowers. All offspring in the first generation (F1) have purple flowers. When two F1 plants are crossed, the second generation (F2) shows both purple and white flowered plants. Explain why white-flowered plants reappear in the F2 generation, referring to the behaviour of alleles during gamete formation and fertilisation.

> OCR A-Level Biology A (H420) — 6.2 Patterns of inheritance · Explain · 4 marks

> In pea plants, flower colour is controlled by a single gene with two alleles. The allele for purple flowers (A) is dominant over the allele for white flowers (a). The original true-breeding parents were crossed to produce the F1 generation, and two F1 plants were then crossed with each other to produce the F2 generation.

## Mark scheme (4 marks)

1. F1 plants are heterozygous, carrying both the dominant and recessive alleles
2. During meiosis, alleles separate so each gamete carries only one allele for flower colour
3. F1 plants produce two types of gametes in equal proportions: gametes carrying A and gametes carrying a
4. Random fertilisation allows two recessive alleles to combine, producing homozygous recessive (aa) offspring with white flowers

## Key terms

- [true-breeding](https://www.gradenine.co.uk/glossary/true-breeding)
- [alleles](https://www.gradenine.co.uk/glossary/alleles)
- [gamete](https://www.gradenine.co.uk/glossary/gamete)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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-student-crosses-two-true-breeding-pea-25c497f9) · Published by Druglandscape Ltd.