'Nonisjunction' is the failure of chromosome pairs to separate properly during cell division. This could arise from a failure of homologous chromosomes to separate in meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis. The result of this error is a cell with an imbalance of chromosomes. When a single chromosome is lost (2n-1), it is called a monosomy, in which the daughter cell(s) with the defect will have one chromosome missing from one of its pairs. When a chromosome is gained, it is called trisomy, in which the daughter cell(s) with the defect will have one chromosome in addition to its pairs.The word nondisjunction means "not coming apart". Examples of nondisjunction: Down syndrome, Triple-X syndrome, Klinefelter's Syndrome, Turner's Syndrome