Trisomy 18 diagnosis. Fluorescence in situ hybrid- isation (FISH) micrograph of Trisomy 18 chromoso- mes (green) in a cell's nucleus (blue). The FISH technique enables individual chromosomes within the nucleus to be tagged with a fluorescent dye. Here, three copies of chromosome 18 are seen in the nucleus. In a healthy human, each nucleus has two copies of every chromosome (the sex pair, here an XX, has been highlighted pink). Chromosomes are the parts of a nucleus responsible for carrying the genetic code. Trisomy 18 causes severe deformity and mental retardation. Few affected infants live over a year. Magnification unknown.