Huntington's disease is a genetic disorder caused by a dominant gene mutation. A person who has one copy of the dominant allele for Huntington's disease will develop the condition. Explain how a gene mutation can lead to a genetic disorder such as Huntington's disease, and why having just one copy of the dominant allele is enough to cause the condition.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Huntington's disease affects the nervous system and causes progressive loss of muscle control. It is caused by a dominant allele that can be inherited from one parent.
Model answer (5 marks)
A gene mutation is a change in the DNA/base sequence of a gene.
The mutation alters the sequence of amino acids in the protein produced, or produces a different protein.
The altered protein cannot carry out its normal function, or functions incorrectly.
Because the allele is dominant, its effect masks the normal recessive allele; only one copy of the dominant allele is required for the abnormal protein to be produced.
Thus, even if only one parent passes on the dominant allele, the heterozygous offspring will develop Huntington's disease.
The mutation alters the sequence of amino acids in the protein produced, or produces a different protein.
The altered protein cannot carry out its normal function, or functions incorrectly.
Because the allele is dominant, its effect masks the normal recessive allele; only one copy of the dominant allele is required for the abnormal protein to be produced.
Thus, even if only one parent passes on the dominant allele, the heterozygous offspring will develop Huntington's disease.
Examiner tips
- Use the exact phrase "dominant allele" and explain masking of recessive allele.
- Show the chain: mutation → altered protein → loss of function → disease.
- Include that only one copy is needed because the allele is dominant.
- Keep answer concise and use the terminology from the mark scheme.
Common mistakes
- Confusing a recessive with a dominant allele; forgetting that one copy is enough.
- Not explaining how the mutation changes the protein sequence.
- Using vague terms like "gene change" without linking to protein function.
Mark scheme (5 marks)
- A gene mutation is a change in the DNA/base sequence of a gene
- The mutation changes the sequence of amino acids in the protein produced / leads to a different protein being made
- The altered protein cannot carry out its normal function / the protein does not function correctly
- The dominant allele masks the effect of the recessive allele / only one copy of the dominant allele is needed to express the condition
- Therefore even if only one parent passes on the dominant allele, the offspring will be affected / a heterozygous person will still develop Huntington's disease
Key terms in this question
gene mutation · dominant allele · genetic disorder
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →