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Indirect testing for genetic markers in a family that has one or several patients
- when there are a number of genes that can each cause the disease - the gene, or
most of the genes not having been located / identified / mapped - autosomal dominant
diseases
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This is not a direct test, but a comparative one, which is called linkage analysis.
In genetic linkage studies, alleles of each of the genes being tested (an allele
is one member of the pair of genes at a given disease locus) are compared between
the patients and the healthy family members. The allele that is present only in
the patients but in none of the healthy family members - i.e. the -shared allele-
of these patients - is the one in which the mutation responsible for the disease
is located. If the -shared allele- is found, it will be possible to test other family
members without identifying the mutation itself, and prenatal testing by amniocentesis or chorionic villus sampling can be carried
out. This is done by looking for the -shared allele- in the
DNA of the affected individuals in the family.
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Because the location of the disease-causing genes, or most of the genes, has yet
to be identified, the test cannot be completed - it takes a long time and will not
be completely reliable.
If there are a number of genes that can cause the disease, the problem is even more
severe.
However, in very large families with a large number of patients (at least 7), an
attempt can be made.
This can be done by performing a comparative test for each of the regions in the
genome (or candidate areas).
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Only in the locus containing the gene responsible for the disease will there be
complete correlation between the genotype and the phenotype - in other words, the
-shared allele- will be found in all the patients, but in none of the healthy relatives.
In the other loci, complete genotype-phenotype correlation will not be found.
The test requires comparison of a number of affected and healthy members of the
same family. The test is only reliable if DNA from several affected individuals
in the family is already available, or if it can be obtained from a blood (or other)
sample. 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 more candidate genes there are the more complex
and difficult it is to perform the test, as a new gene must in fact be identified.
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