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. 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 CF,
there is a one in four, or 25 percent, chance with each subsequent
pregnancy for another child to be born with CF. This means that there is a
three out of four, or 75 percent chance, for another child to not have CF.
The birth of a child with CF 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 CF. 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 CF. This gene is quite large and
complex. Over 900 different mutations in this gene have been found which
cause CF.
The risk for having a mutation in the gene for CF depends on your
ethnic background (for persons without a family history of 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 most common
mutations, and most labs test for anywhere from 30 to 100 total mutations.
The detection rate depends on the person's ethnic background. In general,
the detection rate for the Caucasian population is around 90 percent, 97
percent or more for the Ashkenazi population, 57 percent for Hispanics, 75
percent for African-Americans, and 30 percent for Asians. The detection
rate differs because CF is more common in certain geographical areas and
certain populations of the world. The diagnosis is confirmed by another
test call the sweat chloride test.
There are many people with CF 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 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, or whose partner is a known carrier of CF or
affected with CF.
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