# Explain what is meant by the terms homozygous and heterozygous, and describe how a homozygous dominant individual differs in phenotype from a heterozygous individual when the dominant allele is present.

> Pearson Edexcel International GCSE Biology (4BI1) — 3.2 Inheritance · Explain · 4 marks

> In pea plants, the allele for tall plants (T) is dominant over the allele for short plants (t). A homozygous dominant plant (TT) and a heterozygous plant (Tt) are grown side by side.

## Mark scheme (4 marks)

1. Homozygous means having two identical alleles of a gene (e.g. TT or tt)
2. Heterozygous means having two different alleles of a gene (e.g. Tt)
3. The dominant allele is always expressed / determines the phenotype in both TT and Tt individuals
4. Therefore there is no difference in phenotype between TT and Tt — both are tall

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Biology (4BI1)](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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