A gain or loss in the number of chromosomes from the normal 46 is called aneuploidy. A common form of aneuploidy is trisomy, or the presence of an extra chromosome in cells
Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with any number of complete chromosome sets is called a euploid cell.
