Indirect DNA studies
Sometimes, the error in the gene responsible for causing a disease has not been identified. In these cases "markers" are used to find out whether a person has inherited the crucial region of the genetic code that is passing through the family with the disease. Markers are DNA sequences located close to the area of interest which are so close that they are almost always inherited together with the disease. When markers are this close to a gene where they are inherited together, they are said to be "linked." In this way, if someone has the set of linked markers, they will also have the disease-causing gene. For this reason, these types of studies are also called "linkage studies."
The family member with the disorder in question is studied first (as in direct DNA analysis) and the pattern of their markers is compared to other relatives. If there is an exact match, then those relatives are also likely to have the faulty gene.
The accuracy of linkage studies depends on how close the markers are to the faulty gene. In some cases, a reliable marker is not available and the test, therefore, cannot give any useful information to the healthy family members. In many cases, several family members are needed to establish the most accurate set of markers to determine who is at risk for the disease in the family. Linkage studies may take many weeks to complete because of the complexity of these studies.