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Genetics of Cystic Fibrosis

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Updated June 30, 2008

You may be surprised to learn that cystic fibrosis (CF) is not a rare disease. Effecting about 30,000 people in the United States and 100,000 people worldwide, it is the most common autosomal recessive inherited disease among Caucasians. The disease also occurs in other races, but much less frequently.

What Does Autosomal Recessive Mean?

All human beings have 22 pairs of non sex-linked chromosomes and two sex-linked chromosomes. The 22 non sex-linked pairs are called autosomal chromosomes. The gene that is defective in cystic fibrosis occurs in the seventh pair of chromosomes. Since it occurs in one of the first 22 pairs of chromosomes, the cystic fibrosis defect is autosomal. It is not sex-linked, so the disease can occur in either gender.

Recessive means that both chromosomes in a pair must be defective in order to have the disease. If a person has only one defective chromosome, he or she will be a carrier but will not actually have the disease.

What Gene is Defective in People with CF?

Cystic fibrosis is caused by a defect in the cystic fibrosis transmembrane regulator (CFTR) gene. There are more than 1200 known mutations of the CFTR gene that can cause cystic fibrosis.

How Can a Child Inherit CF If Nobody Else in the Family Has It?

Since CF is a recessive trait, a child must receive two defective genes in order to be born with the disease. As with all chromosome pairs, one is inherited from the mother and one is inherited from the father. This means that both parents must carry the cystic fibrosis trait or have CF themselves in order to have a baby with CF. People who are carriers have only one defective gene. They will not have CF and will not have any symptoms. About 4% of all Caucasians are CF carriers. If both parents are carriers there is a 25% chance that each parent will pass on their defective gene, meaning their baby will have CF.

There may be many generations of carriers in a family without anybody ever having the disease. Often, people do not know they are carriers until their baby is born with CF. Genetic testing is available to determine if a person carries the CF defect, but it is not usually done unless a couple has reason to suspect that they may be carriers.

Sources:
Boyle, MP, MD. “Adult Cystic Fibrosis.” Journal of the American Medical Association. 2007 298:1787-1793. 17 June 2008.

The Genetics of Cystic Fibrosis. University of Virginia Health System. November 7, 2005. http://www.healthsystem.virginia.edu/uvahealth/peds_respire/cfgenetc.cfm

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