Cystic fibrosis is a genetic condition caused by a faulty gene (known as the CFTR gene). The CFTR gene normally creates a protein that moves salt and water out of a cell.

If the CFTR gene is defective, it results in a build-up of thick, sticky mucus in the body's tubes and passageways. These blockages damage the lungs, digestive system and other organs, resulting in inflammation (swelling) and, in the lungs, repeated infections.

How the CFTR mutation is passed through families

Genes come in pairs. You inherit one set of genes from your mother and one set from your father.

To develop cystic fibrosis you would have to inherit the faulty gene from both your mother and your father.

In the UK, it is thought that one person in every 25 carries the faulty gene for cystic fibrosis, which is why it is a relatively common genetic condition. A carrier is usually completely healthy and has no symptoms of cystic fibrosis.

If two carriers of the faulty gene have a baby, there is:

• a one-in-four chance that the child will not inherit either of the faulty genes (the child will not have cystic fibrosis and will not be a carrier of the condition)
• a one-in-two chance that the child will inherit one copy of the faulty gene from either their father or mother (the child will not have cystic fibrosis but will be a carrier of the condition)
• a one-in-four chance that the child will inherit both copies of the faulty gene (the child will have cystic fibrosis)

Chromosome

Chromosomes are the parts of a body cell that carry genes. A human cell usually has 23 pairs of chromosomes.

Gene

Genes contain information that you inherit from your parents, such as eye or hair colour. They are carried by chromosomes.