FISH also known as Fluorescence In Situ Hybridization, is a technique for examining the number of selected chromosomes. The technique involves adding a probe for a specific DNA sequence or chromosome that is attached to a colored marker. The marker is visible under a special fluorescence microscope.
When examining the number of chromosomes in an embryo, it would normally be expected to see two of these markers if the number of chromosomes is normal. An extra marker would be visible if an extra chromosome (trisomy) were present and only one marker if the cell was missing a chromosome (monosomy).
When the chromosomes are normal, two of each fluorescence marker are able to be seen. When the cell has an abnormal number of chromosomes (aneuploidy) then there are extra chromosomes. In this case, one extra green and one extra cyan colored marker.
One of the limitations of FISH is that only 12 of the 24 chromosomes can be tested for any particular cell. As part of the test, probes are applied in groups of four, examined, and then replaced with four more probes and the reexamined again with four more probes and reexamined yet another time. This series of steps can only be performed only so many times before becoming inaccurate.