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Indirect testing for genetic markers in a family that has a one or more patients - when there is only one gene that can cause the disease - X-linked diseases

This is not a direct test, but a comparative one. The alleles (an allele is one member of the pair of genes at a given disease locus) that have been transmitted by the parents to the affected child are compared in order to establish which of the mother-s two X chromosomes the affected child has inherited. If the mother is a carrier of an X-linked disease, she has a normal gene on one of her X chromosomes and an abnormal gene on the other. Such a woman has a 50% chance in every pregnancy of transmitting the abnormal gene to her offspring. This means that 50% of her sons will be affected and 50% healthy, and 50% of her daughters will be carriers and 50% will not.

So without identifying the mutation itself, the patient-s siblings can be tested, and prenatal testing by amniocentesis or chorionic villus sampling can be carried out, by looking for the -shared alleles- in the DNA of the affected individuals in the family.

It is also possible to establish which of the women in the family are carriers. The problem is that if there is only one affected child, the mother is not necessarily a carrier - this has to be established in genetic counseling.

If there are a number of patients, especially from different generations, it can be assumed with certainty that there are a number of carriers, in which case the test for carrier status is reliable.

For some of the X-linked diseases, carrier status can be ascertained by a biochemical or other test that may reveal mildly affected females. A female carrier can use this indirect test for reliable prenatal diagnosis in each pregnancy. In these cases, it is possible to carry out the test if there is DNA available from at least one patient or if the patient has healthy brothers.

The cooperation of the family members is required - they must be examined by a physician in order to ascertain whether they have signs of the disease, and must agree to give a blood sample. The test is relatively simple and rapid compared to tests for reading the full sequence of the gene. It is reliable, but this indirect method always has a small risk for errors of about 1% depending on the genetic counseling given.
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