Non-disjunction ("not coming apart") is the failure of chromosome pairs to separate properly during meiosis stage 1 or stage 2, specifically in the anaphase. 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. Such a cell is said to be aneuploid. Loss of a single chromosome (2n-1), in which the daughter cell(s) with the defect will have one chromosome missing from one of its pairs, is referred to as a monosomy. Gaining a single chromosome, in which the daughter cell(s) with the defect will have one chromosome in addition to its pairs is referred to as a trisomy.
In the event that an aneuploidic gamete is fertilized, a number of syndromes might result. The only known survivable monosomy is Turner syndrome, where the individual is monosomic for the X chromosome. Examples of trisomies include Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13).
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