Cystic fibrosis is a genetic disorder caused by a gene mutation. Explain how a gene mutation can lead to a person having cystic fibrosis, and describe how the condition is inherited.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cystic fibrosis affects the cell membranes of cells lining the lungs and digestive system, causing a build-up of thick, sticky mucus. It is caused by a recessive allele.
Model answer (5 marks)
A gene mutation is a change in the DNA sequence of a gene.
The mutation in the CFTR gene produces a faulty or non‑functional CFTR channel protein.
The defective CFTR protein prevents chloride ions from moving across the cell membrane, causing thick, sticky mucus to build up in the lungs and digestive tract.
Cystic fibrosis is inherited as a recessive allele; a person must inherit two copies of the mutant allele (one from each parent) to develop the disease.
A carrier has one mutant allele and one normal allele, so they do not show symptoms. Two carrier parents can each pass the mutant allele to their child, giving a 1 in 4 (25%) chance that the child will inherit two mutant alleles and have cystic fibrosis.
The mutation in the CFTR gene produces a faulty or non‑functional CFTR channel protein.
The defective CFTR protein prevents chloride ions from moving across the cell membrane, causing thick, sticky mucus to build up in the lungs and digestive tract.
Cystic fibrosis is inherited as a recessive allele; a person must inherit two copies of the mutant allele (one from each parent) to develop the disease.
A carrier has one mutant allele and one normal allele, so they do not show symptoms. Two carrier parents can each pass the mutant allele to their child, giving a 1 in 4 (25%) chance that the child will inherit two mutant alleles and have cystic fibrosis.
Examiner tips
- Use the exact terms ‘gene mutation’, ‘faulty/non‑functional protein’, ‘recessive allele’, ‘carrier’, and ‘1 in 4 chance’. Show the causal chain from mutation to protein defect to symptoms. Mention that two recessive alleles are required for the disease.
- Keep the answer concise – 5 marks, so one sentence per point is sufficient.
Common mistakes
- Confusing dominant with recessive inheritance. Using vague phrases like ‘gene change’ instead of ‘DNA sequence change’. Omitting the 1 in 4 probability or the carrier status.
Mark scheme (5 marks)
- A gene mutation is a change in the base/DNA sequence of a gene
- The mutation leads to a faulty/non-functional protein (channel protein/CFTR) being produced
- Cystic fibrosis is caused by a recessive allele, so a person must inherit two copies of the recessive allele to have the condition
- A person with one recessive allele and one dominant allele is a carrier and does not show symptoms
- Two carrier parents can pass on the recessive allele to offspring, giving a 1 in 4 (25%) chance the child will have cystic fibrosis
Key terms in this question
gene mutation · cystic fibrosis
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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