Cystic fibrosis

Membranes, Proteins, DNA and Gene Expression · Inheritance, cystic fibrosis and genetic screening · note 4 of 6

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In short

Cystic fibrosis is caused by a recessive mutation in the gene for the CFTR protein, a chloride ion channel in cell-surface membranes. The faulty protein reduces the movement of chloride ions and water out of cells, so mucus is thick and sticky. This blocks and damages the gaseous exchange, digestive and reproductive systems.

Cystic fibrosis is a genetic disorder caused by a recessive allele of the CFTR gene, on chromosome 7. Someone with the condition has two copies of the faulty allele; carriers (heterozygous) have one copy and do not have symptoms. The commonest mutation is a deletion of three bases, which removes one amino acid from the protein.

From mutation to symptoms

The gene mutation changes the primary structure of the CFTR protein, so it folds into the wrong tertiary structure.
The faulty protein does not work as a chloride ion channel in the cell-surface membrane (or does not reach the membrane).
Chloride ions are not secreted out of the epithelial cells, so water does not move out by osmosis (the water potential gradient is reduced).
The mucus on the surface of the epithelium is therefore thick and sticky, instead of thin and watery.

Effects of cystic fibrosis on body systems
System	Effect
Gaseous exchange	Thick mucus blocks the airways and traps bacteria, which cannot be removed by cilia. This causes repeated lung infections, inflammation and damage, reducing the surface area for gas exchange and making breathing difficult. Less oxygen reaches the blood.
Digestive	Thick mucus blocks the pancreatic duct, so digestive enzymes (amylase, lipase and protease) cannot reach the small intestine. Food is poorly digested and absorbed, which causes poor growth and malnutrition. Mucus may also block the intestine.
Reproductive	In males, the tubes (vas deferens) that carry sperm may be blocked or missing, causing infertility. In females, thick mucus in the cervix can prevent sperm from reaching the egg, reducing fertility.

Exam tip:

To score full marks on 'how does a mutation cause these symptoms', follow the chain: gene mutation → faulty channel protein → less chloride and water secretion → thick mucus → blockage of the specific system.

Flow diagram from a CFTR gene mutation to a faulty chloride channel, less water leaving by osmosis and thick sticky mucus, branching to effects on the gaseous exchange, digestive and reproductive systems, with normal and faulty chloride channels drawn in a membrane. — How a CFTR mutation leads to **thick sticky mucus** and the symptoms of cystic fibrosis.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between an allele and a gene?

A gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide. An allele is one of the different forms of that gene. A person has two alleles of each gene, one from each parent, and these may be the same or different.

How do you tell from a pedigree diagram whether an allele is dominant or recessive?

If two unaffected parents have an affected child, the allele for the condition is recessive and both parents are carriers. If two affected parents have an unaffected child, the allele is dominant and both parents are heterozygous. Always name the individuals and give their genotypes.

Why does cystic fibrosis cause problems in the lungs, digestive system and reproductive system?

The faulty CFTR chloride channel means less water moves out of cells by osmosis, so mucus is thick and sticky. It blocks the airways, causing infections, blocks the pancreatic duct so enzymes cannot reach the intestine, and blocks the tubes that carry sperm.

All 5 questions on Inheritance, cystic fibrosis and genetic screening