Cystic fibrosis

Causes of cystic fibrosis

Cystic fibrosis is caused when the gene that controls the movement of salt and water in and out of the cells is faulty.

For a child to have cystic fibrosis, both their parents must carry the faulty gene. Genes come in pairs: one set is inherited from the mother and one set from the father. A child will only have cystic fibrosis if they inherit 2 faulty genes – one from their mother and one from their father.

Parents don’t always have cystic fibrosis themselves, but they can be carriers of the faulty gene. This means they have one faulty gene and one normal gene. Because you only have cystic fibrosis when you have 2 faulty genes, people with only one faulty gene have no symptoms. About 1 in 25 of us may carry the gene without knowing it.

If both parents carry the faulty gene, there is a 1 in 4 chance their child will have cystic fibrosis.

Last medically reviewed: September 2016. Due for review: September 2019

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.