Definition: A trisomy is a condition that occurs when a person has three copies of a chromosome instead of two. Most people have 23 pairs of chromosomes, for a total of 46 chromosomes in each cell in the body. Trisomies occur when there are 47 chromosomes instead. Trisomies can cause severe health problems or even death. An example of a trisomy is Down syndrome, a chromosomal condition where people have an extra chromosome 21.