In genetics, a centimorgan or map unit is a unit for measuringgenetic linkage. It is defined as the distance between chromosome positions 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. However, it is not a true physical distance.
Relation to physical distance
The number of base pairs to which it corresponds varies widely across the genome and it also depends on whether the meiosis in which the crossing-over takes place is a part of oogenesis or spermatogenesis. One centimorgan corresponds to about 1 million base pairs in humans on average. The relationship is only rough, as the physical chromosomal distance corresponding to one centimorgan varies from place to place in the genome, and also varies between men and women since recombination during gamete formation in females is significantly more frequent than in males. Morton et al. calculated that the female genome is 4782 cM long, while the male genome is only 2809 cM long. Plasmodium falciparum has an average recombination distance of ~15 kb per centimorgan: markers separated by 15 kb of DNA have an expected rate of chromosomal crossovers of 0.01 per generation. Note that non-syntenic genes are inherently unlinked, and cM distances are not applicable to 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 J. B. S. Haldane's map function, in which the number of chromosomal crossovers is distributed according to a Poisson distribution, a genetic distance of d 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. The probability of recombination is approximately d/100 for small values of d and approaches 50% as dgoes to infinity. The formula can be inverted, giving the distance in centimorgans as a function of the recombination probability:
