Variations in our genes, referred to as mutations, impact the body differently. Some mutations result in a failure of the corresponding protein to function or be produced. Cystic Fibrosis is caused by a mutation in the CFTR gene, which encodes the protein responsible for controlling salt and fluid flow in and out of cells. This leads to abnormal, thick mucus that obstructs airways, liver and pancreas passages, and causes high salt concentration in sweat. 

Cystic Fibrosis is a recessive condition, meaning an individual must possess two mutated copies of the CFTR gene to develop CF. If only one copy is mutated and the other is normal, the person is a CF carrier but does not have CF.

CF carriers can transmit their mutated CFTR gene to their offspring. The probability of having a child with CF is as follows when two CF carriers conceive:

• A child of two CF carriers has a 25% (1/4) chance of having CF. 

• The chance of the child being a carrier but not having CF is 50% (1/2).

• The chance of the child not having CF or carrying the gene is 25% (1/4). 

Individuals with CF can also transmit their mutated CFTR gene to their offspring. If a person with CF has a child with a CF carrier, the probability is as follows:

• 50 percent (1 in 2) the child will have CF.

• 50 percent (1 in 2) the child will be a carrier but will not have CF.

The offspring of two carriers may themselves be carriers of CF like their parents. It is possible for a family with four children to have none, some, or all of them with CF. The likelihood of each child inheriting CFTR mutations from both parents is equal, regardless of the status of the other siblings. If a person with CF has children with a CF carrier, the children will either have CF or be carriers of the gene.