Genetic mutations may take the form of a repeated piece of genetic code (called a 'duplication' or an 'expansion'), a missing chunk (called a 'deletion'), or a tiny alteration in the genetic code (called a 'point mutation'), depending on the condition.
Different types of mutations are found in different conditions, or sometimes in different families with the same condition. It is usually necessary to find the specific genetic mutation responsible for the condition in a particular family before it can be used to test other members of that family.
DNA is obtained by taking a blood sample. If your relative who has the condition is alive, he or she may be asked to give a blood sample, which can be compared directly with yours.
Linked genetic markers or 'family studies'
Sometimes, if there is no obvious genetic mutation (a duplication, deletion or point mutation) in an affected person, direct genetic tests are not possible/informative. In these cases, 'markers' are used to find out whether someone has inherited the crucial part of the genetic code that carries the genetic mutation within it. These are pieces of DNA, situated close to the mutated gene, which can be used to distinguish it; they are said to be 'linked' to the gene. If you are found to have inherited the same ’linked’ markers as the affected person, you are also likely to have inherited the mutated gene.
The accuracy of this depends on how close the marker is to the mutated gene. Sometimes, a reliable marker is not available, so the test does not give any useful information. In order to use linked markers, DNA samples – from an affected person and from as many other family members as possible – need to be available. This will help to establish which 'markers' are linked to the mutated gene in that particular family.