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Skip Navigation LinksHome Page    >    Tests during the pregnancy    >    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 – autosomal recessive diseases
 
 
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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 - autosomal recessive 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. The parents are both carriers (i.e. each parent carries one normal and one abnormal gene), and there is a 25% risk in every subsequent pregnancy that both will transmit the abnormal allele to their offspring, in which case the fetus will be affected because he has received two copies of the abnormal gene.

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 among the affected individuals in the family. This test is only reliable if DNA from the affected individuals in the family is already available, or if it can be obtained from a blood (or other) sample.

The carrier status of family members, even distant ones, can also be determined provided that they and the patient give blood (or other) samples.

The carrier status of their partners, however, cannot be determined by means of this test.

In these cases it is only possible to carry out the test if DNA is available from at least one patient or if the patient has many healthy siblings.
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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