Advanced Lung Disease Transplant Services

Cystic Fibrosis Risks

The genetics of cystic fibrosis

Cystic fibrosis (CF) is a genetic disease. This means that cystic fibrosis is inherited. A person will be born with cystic fibrosis only if two cystic fibrosis genes are inherited - one from the mother and one from the father. A person who has only one cystic fibrosis gene is healthy and said to be a "carrier" of the disease. A carrier has an increased chance of having a child with cystic fibrosis. This type of inheritance is called "autosomal recessive." "Autosomal" means that the gene is on one of the first 22 pairs of chromosomes which do not determine gender, so that the disease equally affects males and females.

Recessive" means that two copies of the gene, one inherited from each parent, are necessary to have the condition. Once parents have had a child with cystic fibrosis, there is a one in four, or 25 percent chance with each subsequent pregnancy, for another child to be born with cystic fibrosis. This means that there is a three out of four, or 75 percent chance, for another child to not have cystic fibrosis.

The birth of a child with cystic fibrosis is often a total surprise to a family, since most of the time (in eight out of 10 families) there is no previous family history of cystic fibrosis. Many autosomal recessive conditions occur this way. Since both parents are healthy, they had no prior knowledge that they carried the gene, nor that they passed the gene to the pregnancy at the same time.

Genes are founds on structures in the cells of our body called chromosomes. There are normally 46 total, or 23 pairs of chromosomes in each cell of our body. The seventh pair of chromosomes contains a gene called the CFTR (cystic fibrosis transmembrane regulator) gene. Mutations or errors in this gene are what cause cystic fibrosis. This gene is quite large and complex. Over 1,800 different mutations in this gene have been found which cause cystic fibrosis.

The risk for having a mutation in the gene for cystic fibrosis depends on your ethnic background (for persons without a family history of cystic fibrosis):

Ethnic Background Risk of cystic fibrosis Mutation Risk of Child with cystic fibrosis
Caucasian 1 in 29 1 in 2,500-3,500
Hispanic 1 in 46 1 in 4,000-10,000
African-American 1 in 65 1 in 15,000-20,000
Asian 1 in 90 1 in 100,000

Testing for the cystic fibrosis gene can be done from a small blood sample and may also include stool and nasal testing. Common mutations are check for first and tests are then expand if the mutation is not found yet there are clinical concerns.

There are many people with cystic fibrosis whose mutations have not been identified. In other words, all of the genetic errors that cause the disease have not been discovered. Because not all mutations are detectable, a person can still be a cystic fibrosis carrier even if no mutations were found by carrier testing.

Testing for the cystic fibrosis gene is recommended for anyone who has a family member with the disease, or whose partner is a known carrier of cystic fibrosis or affected with cystic fibrosis.

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