# Cystic fibrosis is a genetic disorder caused by a gene mutation. Explain why two parents who do not have cystic fibrosis can have a child with the condition.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 1.5 Gene mutations and genetic disorders · Explain · 5 marks

> Cystic fibrosis is caused by a recessive allele. A person must inherit two copies of this allele to be affected by the disorder.

## Mark scheme (5 marks)

1. The allele that causes cystic fibrosis is recessive
2. Each parent carries one copy of the recessive allele / parents are carriers
3. The parents also carry one dominant (normal) allele, which masks the recessive allele so they do not show symptoms
4. Each parent can pass on either their dominant or their recessive allele to their offspring / there is a chance the child inherits the recessive allele from each parent
5. If the child inherits two copies of the recessive allele (one from each parent), they 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 Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](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/cystic-fibrosis-is-a-genetic-disorder-38da9f16) · Published by Druglandscape Ltd.