# Huntington's disease is a genetic disorder caused by a dominant gene mutation. Explain why a child can develop Huntington's disease even if only one parent carries the mutant allele, and describe how this pattern of inheritance differs from that of a recessive genetic disorder such as cystic fibrosis.

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

> Huntington's disease is a late-onset neurological condition affecting muscle control and cognition. It is caused by a dominant allele on an autosome. Cystic fibrosis is caused by a recessive allele on an autosome.

## Mark scheme (5 marks)

1. The Huntington's disease allele is dominant, so only one copy of the mutant allele is needed for the disorder to be expressed.
2. If one parent is heterozygous for the dominant allele, there is a 50% (1 in 2) chance of passing the mutant allele to each child.
3. In cystic fibrosis, the allele is recessive, so a child must inherit two copies of the mutant allele (one from each parent) to develop the disorder.
4. A person who inherits only one copy of the recessive cystic fibrosis allele is a carrier and does not show symptoms of the disorder.
5. Therefore, unlike Huntington's disease, cystic fibrosis can be passed on by two unaffected carrier parents, and the probability of an affected child is 25% (1 in 4) when both parents are carriers.

## Key terms

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

## 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/huntington-s-disease-is-a-genetic-disorder-75bbb116) · Published by Druglandscape Ltd.