Trisomy is a condition in which a person's cells have three chromosomes in a single set. Normally, a person's chromosomes come in pairs and humans have 23 pairs of these chromosomes (46 in total).
When a person has a trisomy disorder, an additional chromosome is found in one of the pairs of chromosomes. When this happens the individual has a total of 47 chromosomes in his cells. Down syndrome, Edward syndrome, and Patau syndrome are the most common gene abnormalities caused by trisomy disorder.