What would their chances of having a child with cystic fibrosis be if only one parent was a carrier and the other parent was homozygous dominant normal?

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 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 1,000 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):

Ethnic Background	Risk of CF Mutation	Risk of Child with CF
Caucasian	1 in 29	1 in 3300
Ashkenazi/Jewish	1 in 29	1 in 3300
Hispanic	1 in 46	1 in 8000-9000
African-American	1 in 65	1 in 15,000
Asian	1 in 90	1 in 32,000

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.

Click here for a handout on the Genetics of Cystic Fibrosis from the CF Foundation.

URL of this page: https://medlineplus.gov/genetics/understanding/inheritance/riskassessment/

When a genetic disorder is diagnosed in a family, family members often want to know the likelihood that they or their children will develop the condition. This can be difficult to predict in some cases because many factors influence a person's chances of developing a genetic condition. One important factor is how the condition is inherited. For example:

Autosomal dominant inheritance: A person affected by an autosomal dominant disorder
has a 50 percent chance of passing the altered gene to each child. The chance that a child will not inherit the altered gene is also 50 percent. However, in some cases an autosomal dominant disorder results from a new (de novo) variant
that occurs during the formation of egg or sperm cells or early in embryonic development. In these cases, the child's parents are unaffected, but the child may pass on the condition to his or her own children.
Autosomal recessive inheritance: Two unaffected people who each carry one copy of the altered gene for an autosomal recessive disorder
(carriers) have a 25 percent chance with each pregnancy of having a child affected by the disorder. The chance with each pregnancy of having an unaffected child who is a carrier of the disorder is 50 percent, and the chance that a child will not have the disorder and will not be a carrier is 25 percent. If only one parent is a carrier of the altered gene and the other parent does not carry the variant, none of their children will develop the condition, and the chance with each pregnancy of having an unaffected child who is a carrier is 50 percent.
X-linked dominant inheritance: The chance of passing on an X-linked dominant condition
differs between men and women because men have one X chromosome and one Y chromosome, while women have two X chromosomes. A man passes on his Y chromosome to all of his sons and his X chromosome to all of his daughters. Therefore, the sons of a man with an X-linked dominant disorder will not be affected, but all of his daughters will inherit the condition. A woman passes on one or the other of her X chromosomes to each child. Therefore, a woman with an X-linked dominant disorder has a 50 percent chance of having an affected daughter or son with each pregnancy.
X-linked recessive inheritance: Because of the difference in sex chromosomes, the probability of passing on an X-linked recessive disorder
also differs between men and women. The sons of a man with an X-linked recessive disorder will not be affected, and his daughters will carry one copy of the altered gene. With each pregnancy, a woman who carries an altered gene for X-linked recessive has a 50 percent chance of having sons who are affected and a 50 percent chance of having daughters who carry one copy of the altered gene. Females with one gene variant associated with an X-linked recessive disorder typically have no or very mild signs or symptoms of the condition.
X-linked: Because the inheritance pattern of many X-linked disorders is not clearly dominant or recessive, some experts suggest that conditions be considered X-linked rather than X-linked dominant or X-linked recessive. As above, the probability of passing on an X-linked disorder differs between men and women. The sons of a man with an X-linked disorder will not be affected, but all of his daughters will inherit the altered gene and may develop signs and symptoms of the condition. A woman passes on one or the other of her X chromosomes to each child. Therefore, with each pregnancy, a woman with an X-linked disorder has a 50 percent chance of having a child with the altered gene. An affected daughter may have milder signs and symptoms than an affected son.
Y-linked inheritance: Because only males have a Y chromosome, only males can be affected by and pass on Y-linked disorders
. All sons of a man with a Y-linked disorder will inherit the condition from their father.
Codominant inheritance: In codominant inheritance
, each parent contributes a different version of a particular gene, and both versions influence the resulting genetic trait. The chance of developing a genetic condition with codominant inheritance, and the characteristic features of that condition, depend on which versions of the gene are passed from parents to their child.
Mitochondrial inheritance: Mitochondria, which are the energy-producing centers inside cells, each contain a small amount of DNA. Disorders with mitochondrial inheritance
result from variants in mitochondrial DNA. Although these disorders can affect both males and females, only females can pass variants in mitochondrial DNA to their children. A woman with a disorder caused by changes in mitochondrial DNA will pass the variants to all of her daughters and sons, but the children of a man with such a disorder will not inherit the variant.

It is important to note that the chance of passing on a genetic condition applies equally to each pregnancy. For example, if a couple has a child with an autosomal recessive disorder, the chance of having another child with the disorder is still 25 percent (or 1 in 4). Having one child with a disorder does not “protect” future children from inheriting the condition. Conversely, having a child without the condition does not mean that future children will definitely be affected.

Although the chances of inheriting a genetic condition appear straightforward, factors such as a person's family history and the results of genetic testing can sometimes modify those chances. In addition, some people with a disease-causing variant never develop any health problems or may experience only mild symptoms of the disorder. If a disease that runs in a family does not have a clear-cut inheritance pattern, predicting the likelihood that a person will develop the condition can be particularly difficult.

Estimating the chance of developing or passing on a genetic disorder can be complex. Genetics professionals can help people understand these chances and help them make informed decisions about their health.

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