# Explain why a testcross is used in genetics to determine whether an organism displaying a dominant phenotype is homozygous or heterozygous.

> IB DP Biology Standard Level (2023 syllabus) — D3.2 Inheritance · Explain · 4 marks

## Mark scheme (4 marks)

1. A testcross involves crossing the organism of unknown genotype with a homozygous recessive individual.
2. If the unknown organism is homozygous dominant, all offspring will display the dominant phenotype.
3. If the unknown organism is heterozygous, approximately half the offspring will display the recessive phenotype.
4. The homozygous recessive parent can only contribute recessive alleles, so any recessive phenotype in the offspring must come from a recessive allele in the unknown parent, thereby revealing its genotype.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-testcross-is-used-cd6ef02e) · Published by Druglandscape Ltd.