# Explain why the phenotypic ratio observed in the offspring of a dihybrid testcross may deviate from the expected 1:1:1:1 ratio when the two gene loci are located on the same chromosome.

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

## Mark scheme (4 marks)

1. Genes on the same chromosome are linked and tend to be inherited together rather than independently.
2. Crossing over / recombination between homologous chromosomes during prophase I of meiosis I can separate linked alleles.
3. Parental (non-recombinant) combinations are more frequent than recombinant combinations, so the two parental phenotypes appear more often than expected in a 1:1:1:1 ratio.
4. The frequency of recombinant offspring reflects the map distance / recombination frequency between the two loci, with more tightly linked genes showing a greater deviation from 1:1:1:1.

## Key terms

- [dihybrid testcross](https://www.gradenine.co.uk/glossary/dihybrid-testcross)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-phenotypic-ratio-observed-ea967515) · Published by Druglandscape Ltd.