Editing Chromosomal inversion (section)

{{Short description|Chromosome rearrangement in which a segment of a chromosome is reversed}}
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An '''inversion''' is a [[chromosome rearrangement]] in which a segment of a chromosome becomes inverted within its original position. An inversion occurs when a [[chromosome]] undergoes a two breaks within the chromosomal arm, and the segment between the two breaks inserts itself in the opposite direction in the same chromosome arm. The breakpoints of inversions often happen in regions of repetitive nucleotides, and the regions may be reused in other inversions.<ref>{{cite journal | vauthors = Corbett-Detig RB, Said I, Calzetta M, Genetti M, McBroome J, Maurer NW, Petrarca V, Della Torre A, Besansky NJ | display-authors = 6 | title = Fine-Mapping Complex Inversion Breakpoints and Investigating Somatic Pairing in the ''Anopheles gambiae'' Species Complex Using Proximity-Ligation Sequencing | journal = Genetics | volume = 213 | issue = 4 | pages = 1495–1511 | date = December 2019 | pmid = 31666292 | pmc = 6893396 | doi = 10.1534/genetics.119.302385 }}</ref> Chromosomal segments in inversions can be as small as 1 kilobases or as large as 100 megabases.<ref>{{cite journal | vauthors = Porubsky D, Sanders AD, Höps W, Hsieh P, Sulovari A, Li R, Mercuri L, Sorensen M, Murali SC, Gordon D, Cantsilieris S, Pollen AA, Ventura M, Antonacci F, Marschall T, Korbel JO, Eichler EE | display-authors = 6 | title = Recurrent inversion toggling and great ape genome evolution | journal = Nature Genetics | volume = 52 | issue = 8 | pages = 849–858 | date = August 2020 | pmid = 32541924 | pmc = 7415573 | doi = 10.1038/s41588-020-0646-x }}</ref> The number of genes captured by an inversion can range from a handful of genes to hundreds of genes.<ref name=":1">{{cite journal | vauthors = Wellenreuther M, Bernatchez L | title = Eco-Evolutionary Genomics of Chromosomal Inversions | journal = Trends in Ecology & Evolution | volume = 33 | issue = 6 | pages = 427–440 | date = June 2018 | pmid = 29731154 | doi = 10.1016/j.tree.2018.04.002 | s2cid = 22051290 | doi-access = free }}</ref> Inversions can happen either through [[ectopic recombination]] between [[Repeated sequence (DNA)|repetitive sequences]], or through chromosomal breakage followed by [[non-homologous end joining]].<ref name=":2">{{cite journal | vauthors = Huang K, Rieseberg LH | title = Frequency, Origins, and Evolutionary Role of Chromosomal Inversions in Plants | journal = Frontiers in Plant Science | volume = 11 | pages = 296 | date = 2020 | pmid = 32256515 | pmc = 7093584 | doi = 10.3389/fpls.2020.00296 | doi-access = free }}</ref>

Inversions are of two types: [[paracentric inversion|paracentric]] and [[pericentric inversion|pericentric]]. Paracentric inversions do not include the [[centromere]], and both breakpoints occur in one arm of the chromosome. Pericentric inversions span the centromere, and there is a breakpoint in each arm<ref name=":0">{{cite journal | vauthors = Kirkpatrick M | title = How and why chromosome inversions evolve | journal = PLOS Biology | volume = 8 | issue = 9 | pages = e1000501 | date = September 2010 | pmid = 20927412 | pmc = 2946949 | doi = 10.1371/journal.pbio.1000501 | doi-access = free }}</ref>.[[Image:Inversioncartoon.gif|thumb|right|250px|A clay model showing why heterozygous inversion loops are visible in [[polytene chromosome]] preparations]]
[[Image:a_arm_inversion.jpg|thumb|right|250px|An inversion loop in the A arm of a chromosome from an ''[[Axarus]]'' species midge]]

Inversions usually do not cause any abnormalities in carriers, as long as the rearrangement is balanced, with no extra or missing DNA. However, in individuals which are [[heterozygous]] for an inversion, there is an increased production of abnormal [[chromatids]] (this occurs when crossing-over occurs within the span of the inversion). This leads to lowered fertility, due to production of unbalanced gametes. Inversions do not involve either loss or gain of genetic information; they simply rearrange the linear DNA sequence.