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
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).
