Nondisjunction

'Nondisjunction' 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.

Examples of nondisjunction: Down's Syndrome, Triple-X syndrome, Klinefelter's Syndrome, Turner's Syndrome

The following diagram shows the two possible types of nondisjunction in meiosis: N.B. "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)

2n(4c)         Duplicated chromosomes in diploid cell. DNA content (c-value) is given in brackets. /        \        Schematic of nondisjunction in meiosis I.      n+1         n-1 (2c)       (2c) /  \       /   \   n+1   n+1   n-1    n-1  All gametes are affected by nondisjunction in meiosis I.   (~c)  (~c)  (~c)   (~c)  Two gametes have a single extra chromosome; two gametes are missing a single chromosome.

2n(4c)          Duplicated chromosomes in diploid cell. DNA content (c-value) is given in brackets. /       \         Schematic of nondisjunction in Meiosis II. n         n    (2c)       (2c) / \       /   \         n    n    n+1   n-1    Half of the gametes are affected by nondisjunction in meiosis II. (c) (c)  (~c)  (~c)    One gamete has a single extra chromosome; one gamete is missing a single chromosome. N.B. Normal c-values will change slightly in aneuploidal cells; values are given here to illustrate the major changes in cellular DNA content.