Nondisjunction 481020 219032326 2008-06-13T07:27:26Z SieBot 4005189 robot Adding: [[de:Non-Disjunction]], [[nl:Non-disjunctie]], [[sv: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.The word nondisjunction means "not coming apart". Examples of nondisjunction: [[Down 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. ==External links== * [http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20101/Bio%20101%20Laboratory/Human%20Genetics%201/Nondisjunction.htm Human Genetics] * [http://ghr.nlm.nih.gov/ghr/glossary/nondisjunction Genetics Home Reference] * [http://www.thedoctorslounge.net/studlounge/downdirty/syndromes.htm Other Syndromes] [[Category:Genetics]] [[de:Non-Disjunction]] [[nl:Non-disjunctie]] [[sv:Nondisjunction]]