Nondisjunction occurs when homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis. ... An individual with an error in chromosome number is described as aneuploid, a term that includes monosomy (loss of one chromosome) or trisomy (gain of an extraneous chromosome).
2 sets of Chromosomes are supposed to be separated into each daughter gamete cells but non disjunction occurred that resulted in both sets of chromosomes being isolated into 1 gamete and the other having no chromosome
