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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
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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.
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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.
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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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