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Indirect testing for genetic markers in a family that has a number of patients -
when there is only one gene that can cause the disease - autosomal dominant 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 are common to patients
are compared. The -shared allele- of the patients within the same family is the
one in which the mutation responsible for the disease is located. So without identifying
the mutation itself, additional family members can be tested, and prenatal testing
by amniocentesis or
chorionic villus sampling can be performed, by looking for the -shared allele-
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. In these
cases it is possible to carry out the test if there is DNA from at least two affected
individuals. 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. Its reliability increases the
larger the family is, and the more affected individuals there are in the family,
but this indirect method always has a risk for errors of about 1% depending on the
genetic counseling
given.
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