What is carrier screening?
Carrier screening is a blood test performed on healthy adults to determine whether they carry a recessive gene mutation that could cause disease in their children.
What does it mean to be a carrier?
A carrier is a person who has only one copy of a gene mutation for a recessive disease. A carrier generally has no disease symptoms but risks having an affected child if her partner is also a carrier.
When is carrier screening performed?
Couples planning a family should ideally be screened before they conceive, or as soon as possible after conception, to allow all available reproductive options to be considered.
Because advances in genetic medicine occur quickly, couples should inquire about screening options before every pregnancy.
What does carrier screening involve?
A blood or saliva sample is taken from one or both partners and sent to a lab where the DNA is examined for evidence of genetic mutations.
How long do results take?
Results of carrier screening are usually available in about 2 weeks.
What do the results mean?
If one partner is a carrier of an autosomal recessive genetic disorder but her partner is not, their children will not be affected by the disease. However, there is a 25% chance with each pregnancy that the child will inherit the defective gene and be a carrier.
[autosomal recessive inheritance chart: one parent a carrier]
If both partners are are found to be carriers of the same disorder, there is a 25% risk with each pregnancy that the fetus will be affected by the disease. The chances are 50% that the child will be a carrier and 25% that the child will inherit no mutation.
[autosomal recessive inheritance chart: both parents carriers]
If a gene mutation is detected on one of the woman's X chromosomes, there is a 50% chance that her male children will inherit the condition. Males who inherit an X-linked condition are generally affected and females are unaffected carriers (though sometimes female carriers may show symptoms, depending on the specific disease and/or mutation).
[x-linked inheritance chart: mother carrier. add a note under the charg saying that "X-linked conditions generally follow this pattern, but Fragile X inheritance is more complicated and has some unique features."]
If both partners are identified as carriers of the same disease, or if the mother carries an X-linked condition, a genetic counselor can provide more information and advice about available options.
For couples who are already pregnant, prenatal diagnostic tests are available to discover whether the fetus is affected.