Cystic fibrosis is caused by a faulty gene that a child inherits from both their parents.

The faulty gene affects the movement of salt and water in and out of cells. This, along with recurrent infections, can result in a build-up of thick, sticky mucus in the body's tubes and passageways – particularly the lungs and digestive system.

The condition is present from birth and cannot be caught from someone else who has it.

How cystic fibrosis is inherited

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

To be born with cystic fibrosis, a child has to inherit a copy of the faulty gene from both of their parents.

This can happen if the parents are "carriers" of the faulty gene. This means they don't have cystic fibrosis themselves, but they can have a child with the condition if their partner also carries the faulty gene.

It's estimated that around 1 in every 25 people in the UK are carriers of the faulty gene that causes cystic fibrosis.

If both parents are carriers, there's a:

  • 25% chance that each child they have will not inherit any faulty genes and won't have cystic fibrosis or be able to pass it on
  • 50% chance that each child they have will just inherit one copy of the faulty gene from one parent and be a carrier
  • 25% chance that each child they have will inherit copies of the faulty gene from both parents and will have cystic fibrosis 

This is known as an autosomal recessive inheritance pattern.

Page last reviewed: 21/03/2016

Next review due: 31/12/2018