Centimorgan

In genetics, a centimorgan (abbreviated cM) or map unit (m.u.) is a unit of recombinant frequency for measuring genetic linkage, defined as that 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. 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^[1][2]. 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.

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. Under a model where the number of chromosomal crossovers is 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

$\Pr[\mathrm{recombination}] = e^{-d/100} \sum_{k=1, \mathrm{odd}}^{\infty} \frac{(d/100)^k}{k!} = e^{-d/100} \sinh(d/100) = \frac{1 - e^{-2d/100}}{2}\,.$

This is approximately d / 100 for small values of d and approaches 50% as d goes to infinity.

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.

References

1. ^ NIH ORDR - Glossary - C
2. ^ 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. pp. 396. ISBN 0-7167-4366-3. ""...in humans 1 centimorgan on average represents a distance of about 7.5x10E5 base pairs"" 

• Su X et al. (November 1999). "A genetic map and recombination parameters of the human malaria parasite Plasmodium falciparum". Science 286 (5443): 1351–3. doi:10.1126/science.286.5443.1351. PMID 10558988.