A couple are both carriers of the recessive allele that causes cystic fibrosis. They are expecting a child. Explain why there is a 1 in 4 chance that the child will have cystic fibrosis, and describe how the faulty allele causes the symptoms of cystic fibrosis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cystic fibrosis is a genetic disorder caused by a recessive gene mutation. A person who carries one faulty allele and one working allele is called a carrier. Carriers do not show symptoms of the disorder.
Model answer (5 marks)
Both parents are carriers, so each has one normal allele (A) and one faulty allele (a). Each parent can pass either allele to the child, giving the following possibilities:
1. A from mother + A from father – normal
2. A from mother + a from father – carrier
3. a from mother + A from father – carrier
4. a from mother + a from father – cystic fibrosis
Only one of the four equally likely combinations (4) gives two faulty alleles, so the probability is 1 in 4.
The faulty allele produces a defective CFTR protein. CFTR normally transports chloride ions across epithelial cells. When it is non‑functional, chloride transport is reduced, water follows less, and mucus becomes thick and sticky. This thick mucus blocks airways and digestive ducts, causing breathing problems and poor nutrient absorption.
1. A from mother + A from father – normal
2. A from mother + a from father – carrier
3. a from mother + A from father – carrier
4. a from mother + a from father – cystic fibrosis
Only one of the four equally likely combinations (4) gives two faulty alleles, so the probability is 1 in 4.
The faulty allele produces a defective CFTR protein. CFTR normally transports chloride ions across epithelial cells. When it is non‑functional, chloride transport is reduced, water follows less, and mucus becomes thick and sticky. This thick mucus blocks airways and digestive ducts, causing breathing problems and poor nutrient absorption.
Examiner tips
- Show the Punnett square or list the four combinations to justify the 1/4 probability. Explain the role of CFTR in chloride transport and how its loss leads to thick mucus. Use the term "recessive allele" and "carrier" as expected by the mark scheme.
Common mistakes
- Mixing up dominant and recessive terminology. Failing to explain how the faulty protein causes thick mucus. Using vague phrases like "genetic disorder" without linking to CFTR function.
Mark scheme (5 marks)
- Both parents are carriers, meaning each parent has one dominant (working) allele and one recessive (faulty) allele
- Each parent can pass on either the dominant or the recessive allele to the child
- The child inherits two recessive alleles (one from each parent) to have cystic fibrosis, and this combination has a 1 in 4 probability
- The faulty allele leads to the production of a faulty (non-functional) protein / faulty CFTR protein, resulting in thick, sticky mucus being produced
- Thick, sticky mucus blocks the lungs and/or digestive system, causing breathing difficulties and/or problems with digestion / nutrient absorption
Key terms in this question
recessive allele · carrier · 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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