# A plant breeder crosses two true-breeding varieties of snapdragon. One variety has red flowers and the other has white flowers. All of the F1 offspring have pink flowers. When the F1 plants are allowed to self-fertilise, the F2 generation contains red, pink and white flowered plants. Explain why the F2 generation contains plants with three different flower colours, rather than showing the 3:1 ratio expected from a monohybrid cross involving complete dominance.

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

> Snapdragon flower colour is controlled by a single gene with two alleles. The allele for red pigment production is represented by C^R and the allele for no pigment production is represented by C^W. Each allele contributes to the phenotype independently.

## Mark scheme (4 marks)

1. Identifies that neither allele is dominant over the other / codominance or incomplete dominance is occurring
2. States that the F1 plants are all heterozygous (C^R C^W) and produce pink flowers because both alleles are expressed simultaneously
3. Correctly identifies the three F2 genotypes produced from self-fertilisation of the F1
4. Links each F2 genotype to its phenotype, explaining that the heterozygote produces a distinct (pink) phenotype giving three phenotype classes

## Key terms

- [true-breeding](https://www.gradenine.co.uk/glossary/true-breeding)
- [self-fertilise](https://www.gradenine.co.uk/glossary/self-fertilise)
- [monohybrid cross](https://www.gradenine.co.uk/glossary/monohybrid-cross)
- [complete dominance](https://www.gradenine.co.uk/glossary/complete-dominance)

## 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-breeder-crosses-two-true-breeding-549351cc) · Published by Druglandscape Ltd.