The condition that this question is asking about is something called aneuploidy. When a cell is "euploid," it is normal or "good." The prefix "an-" means essentially "not," "without," or "lacking;" therefore, aneuploid essentially means not good or not normal. When a cell has an extra chromosome or a missing chromosome, it is said to be aneuploid.
In humans, the correct number of chromosomes in a body cell is 46. This is the diploid chromosome number and is written as "2n." The "n" count is 23 chromosomes. That is the haploid number of chromosomes that are present in a sperm or egg cell. Cells that are 2n + 1 or 2n -1 are aneuploid cells.
Aneuploidy is a result of an error that occurs during meiosis. The specific error is called "nondisjunction" and occurs when a pair of homologous chromosomes does not separate during meiosis 1 or sister chromatids do not separate during meiosis 2. During meiosis 1 and 2, the genetic material lines up across the cell's equator, and the spindle fibers are supposed to pull an equal amount of genetic material to opposite sides of the cell. If the homologous pairs or sister chromatids do not separate, then some of the resulting gametes are either short a chromosome (n-1) or contain an extra chromosome (n+1).