aneuploidy
(AN-yoo-PLOY-dee)
A condition in which there are extra or missing chromosomes in some or all the cells in the body. Human cells, except for egg and sperm cells, normally contain 23 pairs of chromosomes, for a total of 46 chromosomes in each cell. With aneuploidy, the number of chromosomes does not equal 46. For example, people with Down syndrome have three copies of chromosome 21 in each cell.