# A scientist is studying petal colour in a species of flowering plant. Petal colour is controlled by a single gene. The allele for purple petals (P) is dominant over the allele for white petals (p). Two plants that are both heterozygous for petal colour are crossed. Explain what is meant by the terms dominant and heterozygous, and predict the expected genotypes and phenotypes of the offspring from this cross, using a Punnett square approach described in words.

> OCR A-Level Biology B — Advancing Biology (H422) — 5.2 Inheritance and variation · Explain · 5 marks

> In a population of flowering plants, purple-petalled plants are more common than white-petalled plants. A scientist crosses two heterozygous purple-petalled plants to investigate how petal colour is inherited.

## Mark scheme (5 marks)

1. Dominant means the allele is always expressed when present (even if only one copy is present / in a heterozygous individual)
2. Heterozygous means the individual has two non-identical alleles of a gene (e.g. Pp)
3. Correct identification of all four possible genotype combinations from the cross: PP, Pp, Pp, pp (accept 1PP : 2Pp : 1pp or equivalent ratio)
4. Three offspring with purple petals (PP and Pp) and one offspring with white petals (pp) — accept 3:1 ratio stated or implied
5. White-petalled offspring (pp) have white petals because they have two copies of the recessive allele / no dominant allele is present

## Key terms

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

## 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-scientist-is-studying-petal-colour-b41feff1) · Published by Druglandscape Ltd.