Nondisjunction

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).

The following diagram shows the two possible types of nondisjunction in meiosis:

2n / \ 2(n+1) 2(n-1) / \ / \ n+1 n+1 n-1 n-1	2n / \ 2n 2n / \ / \ n n n+1 n-1
Schematic of nondisjunction in meiosis I. Duplicated chromosomes in diploid cell (2n). All gametes are affected by nondisjunction in meiosis I. Two gametes have a single extra chromosome; two gametes are missing a single chromosome.	Schematic of nondisjunction in meiosis II. Duplicated chromosomes in diploid cell (2n). Half of the gametes are affected by nondisjunction in meiosis II. One gamete has a single extra chromosome; one gamete is missing a single chromosome.

"n" denotes a cell with a single copy of each chromosome (haploid cell); 2n denotes a cell with two copies of each chromosome (diploid cell)

Though a person of any age can have nondisjunction occur during meiosis, the chances tend to increase with age.^{citation needed}

References

Snustad, D.P., Simmons, M.J.(2006). Principles of Genetics (4th ed.). Hoboken: John Wiley & Sons, Inc.