# A plant species can produce flowers that are either red, pink, or white. A student crossed two pink-flowered plants and obtained offspring in an approximate ratio of 1 red : 2 pink : 1 white. Explain how this ratio arises, naming the type of inheritance involved.

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

## Mark scheme (4 marks)

1. Identifies the type of inheritance as incomplete (incomplete dominance / codominance acceptable only if correctly applied to this phenotypic pattern)
2. States that pink-flowered plants are heterozygous, carrying one allele for red and one allele for white
3. Explains that crossing two heterozygotes produces offspring in the expected 1:2:1 genotypic ratio (homozygous red : heterozygous pink : homozygous white)
4. Links genotype to phenotype by explaining that in heterozygotes neither allele masks the other, so both are expressed, producing the intermediate pink colour

## Key terms

- [ratio](https://www.gradenine.co.uk/glossary/ratio)

## 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-plant-species-can-produce-flowers-d107ccb4) · Published by Druglandscape Ltd.