Genes are made up of a chemical structure called DNA. To look for single gene defects, healthcare providers look at the DNA to see if it has any errors in it. Errors are known as mutations.
DNA is gathered by taking a sample of blood or saliva. Or it is gathered from swabbing the inside of your cheeks (buccal swab). There are 2 ways to then check the DNA:
Direct studies. These are done by looking at the actual gene itself.
Indirect studies. These are done by looking at markers very close to the gene.
Direct DNA studies mean looking directly at a gene to see if there’s an error. Errors in the DNA may include a repeated piece of DNA (duplication), a missing piece of DNA (deletion), or a change in the DNA code (point mutation). Different types of errors cause different disorders.
The first task is to find which type of error occurs in a family. This is done by checking the DNA of the family member with the disorder. The DNA is then compared with other family members. This can help show who else has the disorder or carries the gene for the disorder. This helps the family learn who’s at risk to pass it on to a child.
In some cases, the error in the gene that causes a disorder is not yet known. In these cases, indirect studies can look for markers. Markers are sections of DNA that are almost always near the part of DNA suspected of causing the disorder. The studies look for an area of the genetic code that is passed on through the family with the disorder. When markers are close to a gene and are inherited together, they are called "linked." If someone has the set of linked markers, he or she will also have the gene that causes the disorder. Indirect DNA studies are also known as linkage studies or genetic mapping.
Linkage studies are complex and can take weeks or months to complete. The accuracy of the studies depends on how close the markers are to the gene that causes the disorder. In some cases, a reliable marker is not available. Then the test isn’t helpful to healthy family members. In many cases, several family members need to be tested to find an accurate set of markers and see who’s at risk for the disorder in the family.