Triple X syndrome. From Wikipedia, the free encyclopedia. Triple X syndrome (also known as triplo-X, trisomy X, XXX syndrome, 47,XXX aneuploidy) is a form of chromosomal variation characterized by the presence of an extra X chromosome in each cell of a human female.A mosaic form also occurs where only a percentage of the body cells contain XXX while the remainder carry XX.riple X syndrome is not inherited, but usually occurs as an event during the formation of reproductive cells (ovum and sperm). An error in cell division called nondisjunction can result in reproductive cells with additional chromosomes.