Genes are made up of the chemical DNA. To study genes, you have to analyze the DNA to determine any errors. There are two ways to analyze the DNA:
- Direct studies study the specific gene
- Indirect studies study the genes that affect the target gene.
Direct DNA studies
Direct DNA studies look directly at the target gene for errors. To look for errors, DNA is usually obtained by taking a blood sample. DNA errors include repeated pieces of DNA, such as a trinucleotide repeat; a missing piece of DNA (deletion); or an alteration in the DNA code, called point mutation.
Different errors or mutations are found with different disorders. It is important to find which errors are present within a family by studying family members. Then, the results of their DNA test are used to compare other relatives within the family to determine who else carries the gene for the disease, finding who is at risk to pass it on to the next generation.
Indirect DNA studies
Sometimes, the gene responsible for causing a disease has not been identified. In these cases, genetic markers are used to find out whether a person has inherited the crucial region of the genetic code.
Markers are DNA sequences located close to the area of interest, which are so close they are almost always inherited together with the disease. When markers are this close to a gene, and they are inherited together--they are said to be linked. If someone has the set of linked markers, then they will also have the disease-causing gene. For this reason, these types of studies are also called linkage studies.
The family member with disease-causing gene receives a direct DNA analysis. 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 disease-causing gene.
The accuracy of indirect DNA studies depends on how close the markers are to the faulty gene. A reliable marker might not be available, and the test will not 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 in the family. Indirect DNA studies may take many weeks to complete because of the complexity of these studies.