# A couple are both carriers of the recessive allele that causes cystic fibrosis. They are expecting a child. Explain why there is a 1 in 4 chance that the child will have cystic fibrosis, and describe how the faulty allele causes the symptoms of cystic fibrosis.

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

> Cystic fibrosis is a genetic disorder caused by a recessive gene mutation. A person who carries one faulty allele and one working allele is called a carrier. Carriers do not show symptoms of the disorder.

## Mark scheme (5 marks)

1. Both parents are carriers, meaning each parent has one dominant (working) allele and one recessive (faulty) allele
2. Each parent can pass on either the dominant or the recessive allele to the child
3. The child inherits two recessive alleles (one from each parent) to have cystic fibrosis, and this combination has a 1 in 4 probability
4. The faulty allele leads to the production of a faulty (non-functional) protein / faulty CFTR protein, resulting in thick, sticky mucus being produced
5. Thick, sticky mucus blocks the lungs and/or digestive system, causing breathing difficulties and/or problems with digestion / nutrient absorption

## Key terms

- [recessive allele](https://www.gradenine.co.uk/glossary/recessive-allele)
- [carrier](https://www.gradenine.co.uk/glossary/carrier)
- [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/a-couple-are-both-carriers-of-2bdda281) · Published by Druglandscape Ltd.