means the presence of three (instead of the normal two) chromosomes
of a particular numbered type in an organism. Thus the presence of 3 chromosome 21's is called Trisomy 21. Most trisomies result in a number of birth defects (which are often present in most individuals with a particular extra chromosome). Many trisomies result in early death (if not a miscarriage). A partial trisomy occurs when part of an extra chromosome (instead of a whole one), is attached to one of the other chromosomes. A mosaic trisomy is when not all cells contains extra chromosomal material.
While a trisomy can occur with any of the chromosomes, the most common types (in humans) are: