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Aneuploidy is an abnormal number of individual chromosomes present in the cells of a species (i.e. in humans there are normally 2 copies of each chromosome but aneuploidy could result in 1 or 3+ copies). In humans, aneuploidy is the result of incorrect division in the parental cells when forming gametes. One cause is nondisjunction (see disjunction) in mitosis or meiosis, whereby homologous chromosomes do not separate during division and therefore do not occupy separate daughter cells.

An abnormal number of chromosomes present in a cell effects the phenotype expressed, which, in humans, will cause genetic disorders if the fertilised egg is able to fully develop.

Examples of genetic disorders caused by aneuploidy are Turner Syndrome (monosomy of the X chromosome in females), Down Syndrome (trisomy of chromosome 21), Edwards Syndome (trisomy of chromosome 18), Patau Syndrome (trisomy of chromosome 13), XYY Syndrome (a male with an extra Y chromosome) and Klinefelter Syndrome (males typically XXY, but there can be variations).

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