Cystic fibrosis is an inherited condition caused by a faulty gene. The gene controls the amount of salt and water that can move between cells in your body, and in those with cystic fibrosis, the amounts are uneven.

The faulty gene allows too much salt and not enough water into your cells, which results in a build up of thick, sticky mucus in your body's tubes and passageways. These blockages damage your body's lungs and digestive system, resulting in inflammation and repeated infections.

In order for a child to be born with cystic fibrosis, they must inherit copies of the faulty gene from both their mother and their father. A carrier can be completely healthy and have no symptoms of cystic fibrosis, as they only have one copy of the faulty gene required for the condition to exist.

If two people who both carry the faulty gene have a baby, the chances of the baby having cystic fibrosis are as follows:

• 25% chance that the child will inherit neither copies of the faulty gene - the child does not have cystic fibrosis nor carries the condition,
• 50% chance that the child will inherit only one copy of the faulty gene from either their father or mother - the child does not have cystic fibrosis but is a carrier of the condition, and
• 25% chance that the child will inherit both copies of the faulty gene - the child has cystic fibrosis.