# Cystic fibrosis is a genetic disorder caused by a gene mutation. Explain how a gene mutation can lead to a person having cystic fibrosis, and describe how the condition is inherited.

> Pearson Edexcel A-Level Biology A (9BI0) — 1.5 Gene mutations and genetic disorders · Explain · 5 marks

> Cystic fibrosis affects the cell membranes of cells lining the lungs and digestive system, causing a build-up of thick, sticky mucus. It is caused by a recessive allele.

## Mark scheme (5 marks)

1. A gene mutation is a change in the base/DNA sequence of a gene
2. The mutation leads to a faulty/non-functional protein (channel protein/CFTR) being produced
3. Cystic fibrosis is caused by a recessive allele, so a person must inherit two copies of the recessive allele to have the condition
4. A person with one recessive allele and one dominant allele is a carrier and does not show symptoms
5. Two carrier parents can pass on the recessive allele to offspring, giving a 1 in 4 (25%) chance the child will have cystic fibrosis

## Key terms

- [gene mutation](https://www.gradenine.co.uk/glossary/gene-mutation)
- [cystic fibrosis](https://www.gradenine.co.uk/glossary/cystic-fibrosis)

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](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/cystic-fibrosis-is-a-genetic-disorder-58ba5cb5) · Published by Druglandscape Ltd.