For most of the traits in our bodies, we have two genes, one inherited from the mother and one from the father. As the result of a change in the gene (mutation), it may not work in the proper way.
Some diseases occur when a person has a change in only one gene. Other diseases (called "recessive") occur when both genes have changed.
A carrier is a person who has a change in one of the genes for a recessive disease. Because two changes are required to produce the disease, the person remains well and unaware of his or her carrier status.
When two carriers have a child, the child has a 25% chance for being affected. When a carrier has a child with a person who is not a carrier, the child is not at risk for the disease.
There are two ways that carrier testing is done. One way is by direct analysis of the genes. The genes are extracted from blood cells. The genes are tested for mutations.
The second way is to test for the amount or activity of a gene product that normally prevents disease. Affected individuals have little, if any, of the gene product. Carriers have a 50% reduction in the gene product.
The choice of test depends on its ability to detect as many carriers as possible.
When carriers decide to have children, the disease status of the baby can be learned before birth by using prenatal diagnosis.
Prenatal diagnosis involves collecting a small sample of placenta between 10 to 12 weeks of pregnancy or a sample of the fluid that bathes the baby between 14 to 20 weeks of pregnancy. (The length of a pregnancy is usually 40 weeks.)
Genetic testing is performed on these samples to learn whether the baby has the disease, is a carrier like the parents, or is not a carrier. Sometimes carriers may decide not to have children with other carriers.
The tests cannot detect all carriers/mutations. Even negative results are reported as likelihood for being a carrier.
There is a small chance that an error may occur, despite the fact that the testing laboratory uses procedures to assure a high level of quality.
In rare instances a result cannot be determined and it may be necessary to obtain another sample. This testing will be performed at no additional cost.
The results of carrier testing are usually available within two to three weeks from when the specimen was collected and if you are found to be a carrier, a genetic counselor will call you.
Your test results will be sent to your physician. If you want to discuss your test result with a genetic counselor, you may do so at any time.