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Skip Navigation LinksHome Page    >    Tests during the pregnancy    >    Indirect testing for genetic markers in a family that has a number of patients – when there are a number of genes that can each cause the disease – all genes having been located / identified / mapped – autosomal dominant diseases
 
 
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Indirect testing for genetic markers in a family that has a number of patients - when there are a number of genes that can each cause the disease - all genes having been located / identified / mapped - autosomal dominant diseases


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.

The problem here is that there is genetic heterogeneity, and the gene responsible for the disease in the family being investigated must be identified out of the several genes that can cause the disease.

This can be done by performing a separate comparative test for the locus of each of the possible genes.

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, where there is a minimum of 6 family members, not including partners, of whom at least half, preferably more, are affected. This 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. 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 more candidate genes there are, the more complex and difficult it is to perform the test. Its reliability increases the larger the family is and the more affected individuals there are, but this indirect method always has a risk for errors of about 1% depending on the genetic counseling given.

 
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