If a genetic disorder runs in my family, what are the chances that my children will have the condition?

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. One important factor is how the condition is inherited. For example:

• A person affected by an autosomal dominant disorder has a 50-percent chance of passing the mutated gene to each child. There is also a 50-percent chance that a child will not inherit the mutated gene (illustration).
• For an autosomal recessive disorder, two unaffected people who each carry one copy of the mutated gene (carriers) have a 25-percent chance with each pregnancy of having a child affected by the disorder. There is a 75-percent chance with each pregnancy that a child will be unaffected (illustration).
• The chance of passing on an X-linked dominant condition differs between men and women because men have one X 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, and his daughters will all inherit the condition (illustration). 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 (illustration).
• 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 mutated gene (illustration). With each pregnancy, a woman who carries an X-linked recessive disorder 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 mutated gene (illustration).

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, in some cases factors such as a person's family history and the results of genetic testing can modify those chances. In addition, some people with a disease-causing mutation 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.

Because estimating the chance of developing or passing on a genetic disorder can be complex, genetics professionals can help people understand these chances and make informed decisions about their health.

For more information about passing on a genetic disorder in a family:

The National Library of Medicine MedlinePlus web site offers information about the chance of developing a genetic disorder on the basis of its inheritance pattern. Scroll down to the section “Statistical Chances of Inheriting a Trait” for each of the following inheritance patterns:

Autosomal dominant

Autosomal recessive

X-linked dominant

X-linked recessive

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