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Indirect testing for genetic markers in a family that has a single patient - 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. It is based on the fact that many
patients with this inheritance pattern are the first ones in their family to be
affected (a -new- mutation). Such individuals are known as probands. Usually in
this situation the mutation is in the gene that has been transmitted to the affected
child by his or her father. A DNA test of the
patient-s parents along with that of the patient can determine which of the patient-s
two genes is the one he inherited from his father (the paternal gene). This gene
probably carries the mutation, and if transmitted to the patient-s offspring it
will probably cause a similar disease (there is a varying confidence margin from
disease to disease, reaching 95% in some cases).
So without identifying the mutation itself, prenatal diagnosis by amniocentesis or chorionic villus sampling can be performed
in each pregnancy of the patient or the patient-s partner. The patient-s parents
and partner must cooperate, as 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 depends on the type of disease and on
the likelihood that the new mutation has occurred in the paternal gene, and this
is determined in genetic
counseling.
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