In genetics, a centiMorgan (abbreviated cM) or map unit (m.u.) is a unit of recombinant frequency for measuring genetic linkage. It is often used to imply 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. The centiMorgan is equal to a 1% chance that a marker at one genetic locus on a chromosome will be separated from a marker at a second locus due to crossing over in a single generation.
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.
Converting centiMorgans into percentages
The way the calculation works is that your total genome in cMs with the Family Finder test is 6770 cM. A half-identical match (such as a parent/child) is 3385 cM. This number has to be doubled to represent both the maternal and paternal sides giving a total of 6770 cM. Matt Dexter explains: "The reason the number is not 6770 or 6800, but rather 68, is that it saves an additional step doing the math to convert an answer to percent. For example, 3385 / 6770 = .5 then as a second step, .5 times 100 = 50%. Using 68 to start with saves the added math step. So (3385 / 6800) * 100 is the same thing as 3385 / 68, which results in = 50%."
- Dexter Matt. Message posted on the ISOGG DNA Newbie list in a thread entitled "DNA Conference Thank You", 13 November 2013.
- Definition of centiMorgan from the FTDNA FAQs
- Definition of centiMorgan from the National Human Genome Research Institute
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