FISH Test - Fluorescence in situ hybridisation.

Used to diagnose genetic disorders in patients. May be used prenatally as a diagnostic test on the foetus when parents may be carriers of genetic disorders. Also used for detection of certain cancers and blood disorders.

Genetic markers are linked to a fluorescent dye which then sticks to a specific sequence of DNA in a chromosome. The presence of these DNA fragments can then be seen as a bright glowing area on the chromosome by using a microscope and UV light. In Williams Syndrome for example, where the marker detects the elastin gene, it only sticks to the DNA in one of a pair of chromosomes (chromosome 7), the other gene being faulty. If Williams syndrome is not present, the marker is seen on both chromosomes.