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 - not all genes
having been located / identified / mapped - X-linked diseases
This is not a direct test, but a comparative one, which is called linkage analysis.
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 in
order to establish which of the mother-s two X chromosomes the affected child has
inherited. If the mother is a carrier of an X-linked disease, she has a normal gene
on one of her X chromosomes and an abnormal gene on the other. Such a woman has
a 50% chance in every pregnancy of transmitting the abnormal gene to her offspring.
This means that 50% of her sons will be affected and 50% healthy, and 50% of her
daughters will be carriers and 50% will not.
In genetic linkage studies, alleles of each of the genes being tested 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
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. It is also possible to establish which women in the family are carriers.
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.
Because not all of the genes have been identified, the test cannot be completed
- it takes a long time and will not be completely reliable.
However, in very large families with a large number of affected individuals (at
least 7), an attempt can be made.
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. 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 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. Sometimes the test is pure
research. The more candidate genes there are, the more complex and difficult it
is to perform the test. If none of the known genes is responsible for the disease
in the family being investigated, a different gene must be sought.