The Genetics of Cystic Fibrosis

Cystic fibrosis (CF) is a genetic disease. This means that CF is inherited. A child will be born with CF only if two CF genes are inherited--one from the mother and one from the father. A person who has only one CF gene is healthy and said to be a carrier of the disease. A carrier has an increased chance of having a child with CF.

Once parents have had a child with CF, they have a 25% chance that each additional child will be born with CF. This means that there is a 75% chance that additional children will not have CF. However, these children may be carriers of the CF gene.

Because most often a family has no history of CF, the diagnosis comes as a surprise to parents. Since both parents are healthy, they didn’t know that they carried the gene, or that they passed the gene to the unborn baby at the same time.

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

According to the Cystic Fibrosis Foundation, if you have no family history of CF, the risk for having a mutation in the gene for CF depends on your ethnic background:

Ethnic Background	Risk of CF Mutation	Risk of Child with CF
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

Screening for cystic fibrosis is part of newborn screening in every state in the U.S. A positive newborn screening is not a diagnosis of CF, but it does mean more testing is done.

A sweat chloride test to measure the amount of salt in your child’s sweat is usually the first test done to try to diagnose CF.

Testing for the CF gene can be done from a small blood sample or from a cheek swab, which is a brush rubbed against the inside of your cheek to obtain cells for testing. Laboratories generally test for the 20 or so most common mutations.

Many people with CF have mutations that 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 CF carrier even if no mutations were found by carrier testing.

Testing for the CF gene is recommended for anyone who has a family member with the disease. It’s also recommended if your partner is a known carrier of CF or affected with CF.