Every person carries two copies of most genes (one copy from the mother and one from the father). A carrier is a person who has a change in one copy of a gene. The carrier does not have the genetic disease related to the abnormal gene. A carrier can pass this abnormal gene to a child. Carrier identification is a type of genetic testing that can determine whether people who have a family history of a specific disease, or who are in a group that has a greater chance of having a disease, are likely to pass that disease to their children. Information from this type of testing can guide a couple's decision about having children.
For many genetic disorders, carrier testing can help determine how likely it is that a child will have the disease:
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If both parents carry the abnormal gene, there is a 1-in-4 (25%) chance that their child will have the disease and a 2-in-4 (50%) chance that their child will be a carrier of the disease (but will not have it). There is also a 1-in-4 (25%) chance that the child will not get the abnormal gene and so will not have the disease nor be a carrier.
If only one parent carries the abnormal gene, the child has a 1-in-2 (50%) chance of being a carrier but almost no chance that he or she will have the disease.
Examples of screening tests to identify carriers for specific genetic disorders include:
Cystic fibrosis carrier screening. This test identifies the most common changes (mutations) in the cystic fibrosis transmembrane regulator (CFTR) gene. Many couples planning to become pregnant have this type of screening to determine whether either or both of them carry a defective CFTR gene.
Tay-Sachs test. This test is used to identify Tay-Sachs carriers. People of Ashkenazi Jewish or French-Canadian descent or those who have a family history of Tay-Sachs disease may choose to be tested to see if they are a Tay-Sachs carrier.