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Carrier testing is performed to determine whether a person carries one copy of an altered gene for a particular disease. The disease may be autosomal recessive, which means that the disease is present in an individual only if two copies of the altered gene are inherited. Couples who both carry the same autosomal recessive gene have a one in four, or 25 percent, chance with each pregnancy to have a child with that disease.
A recessive disease may also be X-linked recessive, which means that the altered gene is located on the X chromosome. Since females have two X chromosomes, and males have one X and one Y chromosome, females can be carriers of a gene on the X but are not affected (provided the other X has the normal copy of the gene). On the other hand, males are usually affected with the disease, if they have the altered gene on their X chromosome (because they do not possess the normal copy of the gene on the Y chromosome). Therefore, carrier testing for X-linked conditions is usually done in females.
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