Editing Karyotype (section)

=== Aneuploidy ===

[[Aneuploidy]] is the condition in which the chromosome number in the cells is not the typical number for the species. This would give rise to a [[chromosome abnormality]] such as an extra chromosome or one or more chromosomes lost. Abnormalities in chromosome number usually cause a defect in development. [[Down syndrome]] and [[Turner syndrome]] are examples of this.

Aneuploidy may also occur within a group of closely related species. Classic examples in plants are the genus ''[[Crepis]]'', where the gametic (= haploid) numbers form the series x = 3, 4, 5, 6, and 7; and ''[[Crocus]]'', where every number from x = 3 to x = 15 is represented by at least one species. Evidence of various kinds shows that trends of evolution have gone in different directions in different groups.<ref>Stebbins, G. Ledley, Jr. 1972. ''Chromosomal evolution in higher plants''. Nelson, London. p18</ref> In primates, the [[great apes]] have 24x2 chromosomes whereas humans have 23x2. [[Human chromosome 2]] was formed by a merger of ancestral chromosomes, reducing the number.<ref>{{cite journal |vauthors=IJdo JW, Baldini A, Ward DC, Reeders ST, Wells RA |title=Origin of human chromosome 2: an ancestral telomere-telomere fusion |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=88 |issue=20 |pages=9051–5 |date=October 1991 |pmid=1924367 |pmc=52649 |bibcode=1991PNAS...88.9051I |doi=10.1073/pnas.88.20.9051|doi-access=free }}</ref>