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.
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).
One centimorgan corresponds to about 1 million base pairs in humans on average. 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.
Relation to the probability of recombination 
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, a genetic distance of centimorgans will lead to an odd number of chromosomal crossovers, and hence a detectable genetic recombination, with probability
Where sinh(·) is the hyperbolic sine function. can be calculated directely using
The probability of recombination is approximately for small values of and approaches 50% as goes to infinity.
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.
- NIH ORDR - Glossary - C
- Matthew P Scott, Paul Matsudaira, Harvey Lodish, James Darnell, Lawrence Zipursky, Chris A Kaiser, Arnold Berk, Monty Krieger (2004). Molecular Cell Biology, Fifth Edition. San Francisco: W. H. Freeman. p. 396. ISBN 0-7167-4366-3. "...in humans 1 centimorgan on average represents a distance of about 7.5x10E5 base pairs"