centimorgan

A unit of genetic distance, used to describe the separation of genes on a chromosome. It dates from the early 20th century, the days of genetic research with fruit flies, before the discovery of DNA and the mapping of genomes using restriction enzymes and other advanced techniques. The terms  “millimorgan” and "sub-millimorgan" are also in use (but not micromorgan). The unit originated in work by Thomas Hunt Morgan in 19 and was refined and named by J. B. S. Haldane in 1919.

One centimorgan is defined as the genetic distance between two loci with a statistically corrected recombination frequency of 1%; the genetic distance in centimorgans is numerically equal to the recombination frequency expressed as a percentage.  Symbol, cM. The centimorgan is now more commonly called a “map unit” (symbol, mu) or locus map unit (symbol, LMU). 

The qualification “statistically corrected” is necessary because at genetic distances greater than about 7 cM, the relationship between recombination frequency and genetic distance is no longer linear. Researchers have developed mathematical models that can correct for this difficulty. 

Other mapping functions are possible. After Haldane's, the most common is that of D. D. Kosambi. Haldane function is purely mathematical; Kosambi's incorporates an empircial term. distinguish between "Haldane centimorgans" or "Kosambi centimorgans"

The centimorgan is not a measure of physical distance, but typically a genetic distance of 1 cM corresponds to a physical distance of roughly one million base pairs. Attempts to assign a physical length to the centimorgan have led to an estimate that it is roughly about 0.003 millimeters. 

History

In 1911, A. H. Sturtevant, an undergraduate working in the geneticist T. H. Morgan's laboratory, had an inspiration: a map of a chromosome showing relative locations of genes could be made using the phenomenon of "crossing over." During meiosis, two homologous chromosomes may link, so that one in each new pair of chromosomes combines part of a chromosome from the mother with part of the father's.  If the break takes place between two genes, a new recombinant gamete is formed, which may be detectable in the appearance of the new organism.  The farther apart the two genes are on the chromosome, the more likely it is that they will recombine.

Recombination obviously involves an element of chance, and in 1919 J. B. S Haldane 

The unit of distance is thus 100 times Morgan's unit. [page 303]

...

It is suggested that the unit of distance in a chromosome as defined above be termed a "morgan," on the analogy of the ohm, volt, etc. Morgan's unit of distance is therefore a centimorgan. [page 305]

Haldane named the morgan for Sturtevant's mentor, Thomas Hunt Morgan (1866–1945), who won a Nobel Prize for his work on the genetics of fruit flies.

A. H. Sturtevant.
The linear arrangement of six sex-linked factors in Drosophila, as shown by their mode of association.
Journal of Experimental Zoology, vol. 14, pages 43-59 (1913).

Avaliable online at www.esp.org.foundations/genetics/classical/holdings/s/ahs-13.pdf 

J. B. S. Haldane.
The combination of linkage values, and the calculation of distances between linked factors.
Journal of Genetics, vol. 8, pages 299-309, (1919).

So far as we have been able to discover, Haldane's article is not yet available online. 

D. D. Kosambi.
The estimation of map distance from recombination values.
Annals of Eugenics, vol. 12, pages 172-175. (1944).

Resources

The National Academy Press makes available its biography of Sturtevant at www.nap.edu/readingroom/books/biomems/asturtevant.pdf . It contains his description of the insight that led to the map unit.

This subject is covered in any good genetics text. For example:

 

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