# Sickle cell disease is a genetic disorder caused by a recessive gene mutation. A person who is heterozygous for the sickle cell allele is described as a carrier. Two carriers have children together. Explain why some of their children may have sickle cell disease, some may be carriers, and some may be unaffected and not carriers.

> Pearson Edexcel A-Level Biology A (9BI0) — 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 mutation results in red blood cells that are an abnormal sickle shape, which can block blood vessels and reduce oxygen transport. The allele for sickle cell disease (s) is recessive to the normal allele (S).

## Mark scheme (5 marks)

1. Both parents are carriers, so each parent has one dominant normal allele (S) and one recessive sickle cell allele (s) / both parents are heterozygous (Ss)
2. Each parent passes one allele to the child / alleles are inherited from each parent during reproduction
3. A child who inherits two recessive alleles (ss) will have sickle cell disease
4. A child who inherits one dominant and one recessive allele (Ss) will be a carrier but unaffected
5. A child who inherits two dominant alleles (SS) will be unaffected and not a carrier / the normal allele is dominant so SS children are completely unaffected

## Key terms

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

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/sickle-cell-disease-is-a-genetic-eb08b559) · Published by Druglandscape Ltd.