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centimorgan
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, loci or markers) for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01. It is often used to infer distance along a chromosome. It is not a true physical distance however.
== Relation to physical distance==
The number of base-pairs to which it corresponds varies widely across the genome (different regions of a chromosome have different propensities towards crossover) and it also depends on if the meiosis where the crossing-over takes place is a part of oogenesis (formation of female gametes) or spermatogenesis (formation of male gamets).
One centimorgan corresponds to about 1 million base pairs in humans on average.〔(NIH ORDR - Glossary - C )〕 To be precise, one centimorgan corresponds in average to 1.09 million base pairs in man and to 0.64 million base pairs in women.〔Morton N E: Parameters of the human genome. Proc Natl Acad Sci U S A. 1991 Sep 1; 88(17): 7474–7476. PMCID: PMC52322〕 ''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-syntenic genes (genes residing on different chromosomes) are inherently unlinked, and cM distances have no meaning between them.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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