Editing Genetic recombination (section)

==Chromosomal crossover==
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{{main|Chromosomal crossover}}
[[Image:Morgan crossover 1.jpg|thumb| [[Thomas Hunt Morgan]]'s illustration of crossing over (1916)]]

In [[eukaryotes]], recombination during [[meiosis]] is facilitated by [[chromosomal crossover]]. The crossover process leads to offspring having different combinations of genes from those of their parents, and can occasionally produce new chimeric [[allele]]s.{{cn|date=March 2024}} The shuffling of genes brought about by genetic recombination produces increased [[genetic variation]]. It also allows sexually reproducing organisms to avoid [[Muller's ratchet]], in which the [[genome]]s of an [[Asexual reproduction|asexual]] [[population]] tend to accumulate more deleterious mutations over time than beneficial or reversing mutations.{{cn|date=March 2024}}

Chromosomal crossover involves recombination between the paired [[chromosome]]s inherited from each of one's parents, generally occurring during [[meiosis]].{{cn|date=March 2024}} During [[Meiosis#Prophase I|prophase I]] (pachytene stage) the four available [[chromatid]]s are in tight formation with one another.{{cn|date=March 2024}} While in this formation, [[Sequence homology|homologous sites]] on two chromatids can closely pair with one another, and may exchange genetic information.<ref name=Alberts>{{cite book | vauthors = Alberts B | title = Molecular Biology of the Cell, Fourth Edition | publisher = Garland Science | location = New York | year = 2002 | isbn = 978-0-8153-3218-3 }}</ref>

Because there is a small probability of recombination at any location along a chromosome, the [[recombination frequency|frequency of recombination]] between two locations depends on the distance separating them.{{cn|date=March 2024}} Therefore, for genes sufficiently distant on the same chromosome, the amount of crossover is high enough to destroy the correlation between alleles.{{cn|date=March 2024}}

Tracking the movement of genes resulting from crossovers has proven quite useful to geneticists. Because two genes that are close together are less likely to become separated than genes that are farther apart, geneticists can deduce roughly how far apart two genes are on a chromosome if they know the frequency of the crossovers.{{cn|date=March 2024}} Geneticists can also use this method to infer the presence of certain genes. Genes that typically stay together during recombination are said to be [[genetic linkage|linked]]. One gene in a linked pair can sometimes be used as a marker to deduce the presence of the other gene. This is typically used to detect the presence of a disease-causing gene.<ref>{{Cite web | title = Access Excellence | work = Crossing-over: Genetic Recombination | publisher = The National Health Museum Resource Center | url = http://www.accessexcellence.org/RC/AB/BC/Genetic_Recombination.php | access-date = February 23, 2011}}</ref>

The recombination frequency between two loci observed is the ''[[crossing-over value]]''. It is the frequency of [[Chromosomal crossover|crossing over]] between two linked [[gene]] loci ([[genetic marker|markers]]), and depends on the distance between the genetic [[Locus (genetics)|loci]] observed. For any fixed set of genetic and environmental conditions, recombination in a particular region of a linkage structure ([[chromosome]]) tends to be constant, and the same is then true for the crossing-over value which is used in the production of [[genetic map]]s.<ref name="RM"/><ref>{{cite book | vauthors = King RC, Stransfield WD | date = 1998 | title = Dictionary of Genetics. | publisher = Oxford University Press | location = New York, Oxford | isbn = 0-19-509442-5 }}</ref>