# Sickle cell disease is a genetic disorder caused by a recessive gene mutation. A person with sickle cell disease has red blood cells that are an abnormal shape. Explain how a gene mutation can lead to the production of abnormal red blood cells, and explain why two parents who are carriers of sickle cell disease have a 25% chance of having a child with sickle cell disease.

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

> Sickle cell disease is caused by a mutation in the gene that codes for haemoglobin, the protein found in red blood cells. Carriers of sickle cell disease have one copy of the mutant allele and one copy of the normal allele.

## Mark scheme (5 marks)

1. A gene mutation is a change in the DNA / base sequence of a gene
2. The changed gene codes for a different / abnormal protein (haemoglobin), so the red blood cells have an abnormal shape
3. Carriers have one recessive allele and one dominant (normal) allele, so they do not have the disease
4. Each parent can pass on either the normal or the mutant allele to the child
5. There is a 1 in 4 / 25% chance the child inherits the recessive allele from both parents, so the child has two copies of the mutant allele and has sickle cell disease

## Key terms

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

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/sickle-cell-disease-is-a-genetic-72640055) · Published by Druglandscape Ltd.