FISH micrograph of abnormal chromosomes. FISH (fluorescence in-situ hybridisation) is a technique that highlights a particular gene or chromosome for analysis. Here, it is being used on whole chromosomes to find abnormalities. Chromosomes contain genes, which are lengths of DNA (deoxyribonucleic acid) that control specific functions in an organism. Humans have 23 pairs of chromosomes. They appear X shaped and are made of a central zone called a centromere (pink), with long extensions called arms (blue) on either side. Lots of abnormalities can be seen in these chromosomes, such as the translocation (swapping) of pieces between the green chromosome and the pink/blue chromosome (upper left).