# A cat breeder is studying eye colour in Siamese cats. Blue eye colour is controlled by a dominant allele (B) and green eye colour is controlled by a recessive allele (b). One parent cat is homozygous dominant and the other parent cat is homozygous recessive. Explain what the genotypes and phenotypes of all offspring from this cross would be, and explain why all the offspring have the same phenotype.

> Edexcel GCSE Biology (1BI0) — 3.4 Inheritance · Explain · 4 marks

## Mark scheme (4 marks)

1. States that both parent genotypes are BB and bb respectively (or correctly identifies both parental gametes as B and b)
2. States that all offspring have the genotype Bb (heterozygous)
3. States that all offspring have blue eyes (dominant phenotype)
4. Explains that all offspring show the dominant phenotype because B is dominant and masks / is expressed over the recessive allele b

## Key terms

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

## Related

- [Revision notes for Edexcel GCSE Biology (1BI0)](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-cat-breeder-is-studying-eye-243a0a9f) · Published by Druglandscape Ltd.