ANEUPLOIDY

=Aneuploidy= Our normal body (autosomal) cells are diploid. This means they carry two versions of each chromosome and thus also two versions of each gene (alleles). However, occasionally an individual can inherit one less or one extra chromosome, this is known as aneuploidy. When an individual inherits an extra chromosome (2n + 1), this is known as trisomy. When they inherit one less chromosome (2n -1), this is known as monosomy. Aneuploidy usually results for a failure of the chromosomes to separate, known as non-disjunction, during meiosis. [image:http://i.imgur.com/mHFOjfm.png] '''Non-Disjunction''' Aneuploidy is the result of a non-disjunction, this can occur at meiosis I or meiosis II. If non-disjunction occurs during meiosis II only half of the resulting gametes are affected. Note that gametes containing an extra chromosome will produce individuals who are trisomy for that chromosome. Gametes that are missing a single chromosome will produce individuals who are monosomy for that chromosome. Aneuploidy resulting from the non-disjunction of chromosome 21 during meiosis is responsible for approx 95% of all down syndrome cases. '''Evolutionary significance''' Most organisms that are affected by aneuploidy are infertile because their chromosomes cannot pair correctly during meiosis. In rare cases, infertility due to aneuploidy can be overcome by genome duplication, restoring fertility by providing each chromosome with a pairing partner during meiosis. Genome duplication following aneuploidy could therefore lead to the evolution of new chromosomal combinations and, depending on how much chromosomal information is lost or duplicated in these events, might ultimately lead to speciation (the development of a new species).