# A plant species can produce flowers that are either red, pink or white. A student crosses two pink-flowered plants and obtains offspring in the ratio 1 red : 2 pink : 1 white. Explain how this ratio arises, and state what this pattern of inheritance is called.

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

## Mark scheme (4 marks)

1. Names the pattern of inheritance as incomplete (or partial) dominance / codominance of both alleles over the other (accept co-dominance only if the student explicitly states both alleles are expressed equally — do not accept dominance alone)
2. States that pink-flowered plants are heterozygous (one allele for red, one for white)
3. Correctly identifies the cross as heterozygous × heterozygous (or gives correct parental genotypes), producing offspring in genotypic ratio 1 homozygous red : 2 heterozygous : 1 homozygous white
4. Links phenotype to genotype by explaining that in heterozygotes neither allele fully masks the other, so an intermediate (pink) phenotype results from the expression of both alleles

## 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-ea0c6923) · Published by Druglandscape Ltd.