# Crossover value

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{{short description|Concept in genetics}}
In [genetics](/source/genetics), the '''crossover value''' is the linked frequency of [chromosomal crossover](/source/chromosomal_crossover) between two [gene](/source/gene) loci ([markers](/source/genetic_marker)). For a fixed set of genetic and environmental conditions, [recombination](/source/genetic_recombination) in a particular region of a linkage structure ([chromosome](/source/chromosome))  tends to be constant and the same is then true for the crossover value which is used in the production of [genetic map](/source/genetic_map)s.<ref name="RM">{{cite book |author=Rieger R. |author2=Michaelis A. |author3=Green M. M.|year=1976|title=Glossary of genetics and cytogenetics: Classical and molecular|publisher=Springer-Verlag|place= Heidelberg - New York|isbn= 3-540-07668-9}}</ref><ref>King R. C., Stransfield W. D. (1998): Dictionary of genetics. Oxford University Press, New York, Oxford, {{ISBN|0-19-50944-1-7}}; {{ISBN|0-19-509442-5}}.</ref>

==Origin in cell biology==
Crossover implies the exchange of chromosomal segments between non-sister [chromatid](/source/chromatid)s, in [meiosis](/source/meiosis) during the production of [gamete](/source/gamete)s. The effect is to assort the [allele](/source/allele)s on parental chromosomes, so that the gametes carry ''recombinations'' of [gene](/source/gene)s different from either parent. This has the overall effect of increasing the variety of [phenotype](/source/phenotype)s present in a population.

The process of non-sister  chromatid exchanges, including the crossover value, can be observed directly in stained [cell](/source/Cell_(biology))s, and indirectly by the presence or absence of [genetic marker](/source/genetic_marker)s on the chromosomes. The visible ''crossovers'' are called ''[chiasma](/source/Chiasma_(genetics))ta''.

The large-scale effect of crossover is to spread [genetic variation](/source/genetic_variation)s within a population, as well  as genetic basis for the selection of the most adaptable [phenotype](/source/phenotype)s. The crossover value depends on the mutual distance of the genetic [loci](/source/Locus_(genetics)) observed. The crossover value is equal to the recombination value or fraction when the distance between the markers in question is short.

==See also==
*[Chromosomal crossover](/source/Chromosomal_crossover)
*[Genetic recombination](/source/Genetic_recombination)

==References==
{{reflist}}

Category:Classical genetics
Category:Cellular processes
Category:Cytogenetics

{{genetics-stub}}

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Adapted from the Wikipedia article [Crossover value](https://en.wikipedia.org/wiki/Crossover_value) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Crossover_value?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.