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| In [[genetics]], a '''centimorgan''' (abbreviated '''cM''') or '''map unit''' (m.u.) is a unit for measuring [[genetic linkage]]. It is defined as the distance between chromosome positions (also termed, [[locus (genetics)|loci]] or [[genetic marker|markers]]) for which the expected average number of intervening [[chromosomal crossover]]s in a single generation is 0.01. It is often used to infer distance along a [[chromosome]].
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| == Relation to physical distance==
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| The number of base-pairs to which it corresponds varies widely across the [[genome]] (different regions of a chromosome have different propensities towards crossover). | |
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| One centimorgan corresponds to about 1 million base pairs in humans on average.<ref>[http://rarediseases.info.nih.gov/Glossary.aspx?acronym=False#C NIH ORDR - Glossary - C]</ref><ref>{{cite book |author=Matthew P Scott, Paul Matsudaira, Harvey Lodish, James Darnell, Lawrence Zipursky, Chris A Kaiser, Arnold Berk, Monty Krieger |title=Molecular Cell Biology, Fifth Edition |publisher=W. H. Freeman |location=San Francisco |year=2004 |pages=396 |isbn=0-7167-4366-3 |oclc= |doi= |quote="...in humans 1 centimorgan on average represents a distance of about 7.5x10E5 base pairs"}}</ref> ''[[Plasmodium falciparum]]'' has an average recombination distance of ~15 kb per centimorgan: markers separated by 15 kb of DNA (15,000 nucleotides) have an expected rate of chromosomal crossovers of 0.01 per generation. Note that non-[[Synteny|syntenic]] genes (genes residing on different chromosomes) are inherently unlinked, and cM distances have no meaning between them.
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| ==Relation to the probability of recombination ==
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| Because genetic recombination between two markers is detected only if there are an odd number of chromosomal crossovers between the two markers, the distance in centimorgans does not correspond exactly to the probability of genetic recombination. Assuming [[Haldane's map function]], where the number of chromosomal crossovers is according to a [[Poisson distribution]],<ref>{{cite web|url= http://www.ag.ndsu.nodak.edu/plantsci/adv_genetics/genetics/mf/mf01.htm|title= Department of Plant Sciences}}</ref> a genetic distance of ''d'' centimorgans will lead to an odd number of chromosomal crossovers, and hence a detectable genetic recombination, with probability
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| :<math>\Pr[\text{recombination}|\text{linkage of }d\text{ cM}] = \sum_{k=0}^{\infty} \Pr[2k + 1 \text{ crossovers}|\text{linkage of }d\text{ cM}]</math>
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| :<math>{} = \sum_{k=0}^{\infty} e^{-d/100} \frac{(d/100)^{2\,k+1}}{(2\,k+1)!} = e^{-d/100} \sinh(d/100) = \frac{1 - e^{-2d/100}}{2}\,,</math>
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| where ''sinh(·)'' is the [[hyperbolic sine]] function. The probability of recombination is approximately ''d''/100 for small values of ''d'' and approaches 50% as ''d'' goes to infinity.
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| The formula can be inverted, giving the distance in centimorgans as a function of the recombination probability:
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| :<math>d=50 \ln\left({\frac{1}{1 - 2 \Pr[\text{recombination}]}}\right)\,.</math> | |
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| ==Etymology==
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| The centimorgan was named in honor of geneticist [[Thomas Hunt Morgan]] by his student [[Alfred Henry Sturtevant]]. Note that the parent unit of the centimorgan, the '''Morgan''', is rarely used today.
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| ==External links==
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| [http://hydrodictyon.eeb.uconn.edu/people/plewis/applets/CrossOver/crossingover.html Demonstration of crossing over], from the home page of the laboratory of Paul O. Lewis in the Department of Ecology and Evolutionary Biology at the University of Connecticut. | |
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| ==References==
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| {{reflist|2}}
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| ===Further reading===
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| *{{cite journal |author=Su X |title=A genetic map and recombination parameters of the human malaria parasite Plasmodium falciparum |journal=Science |volume= 286|issue= 5443|pages= 1351–3|date=November 1999 |pmid=10558988 |doi=10.1126/science.286.5443.1351 |last2=Ferdig |first2=MT |last3=Huang |first3=Y |last4=Huynh |first4=CQ |last5=Liu |first5=A |last6=You |first6=J |last7=Wootton |first7=JC |last8=Wellems |first8=TE}}
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| [[Category:Units of measurement]]
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| [[Category:Genetics]]
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| [[Category:Molecular genetics]]
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