Question

What is cM and where do I find it?

Comments

The only cM I know is  centimorgan which is a measurement used in genetics . Here's a wiki page that explains https://en.wikipedia.org/wiki/Centimorgan

and here in relation to genetic genealogy

http://isogg.org/wiki/CentiMorgan

I did  learn how to calculate it not so long ago for a genetics course (not genealogy) course but it really  isn't something that I can imagine doing very often .

Answer 1

I agree with Helen's answer. It is the length in a single continuous snippet of DNA material as measured by an autosomal (at-) DNA test. You will find it used in chromosome browsers at most testing companies (excepting Ancestry for some reason). It is a uniform measurement that provides the magnitude (size in terms of length) of DNA snippets shared among family members on specific chromosomes in specific locations on the chromosome. In this way it is possible to look for distant relatives - those sharing as little as 7 cM (in any location) as well as parents or siblings sharing as much as half. You will also find it used if you compare your autosomal test results with another kit at GEDmatch.com among their various tools and reports. Matches are usually determined by the number and length (in terms of cM) of DNA snippets you share with another person's atDNA test results. I hope others will correct me if I am wrong!

Comments

 Very small segments are more likely to be identical by state (just chance) than by descent   In 23 and me DNA  relatives, the cut off point for identifying a match is 7cM.  There are also a couple of answers here which explain what  a centimorgan is. http://genealogy.stackexchange.com/questions/3364/what-is-the-significance-of-shared-centimorgans

Answer 2

As others stated out it's an abbreviation for Centimorgan, which was actually named after a guy named Morgan.

A centimorgan is not the same in distance as to how we normally understand it (eg. 100 cm is 1m). The reason is that the centimorgan (and the morgan) is used to show the number of base pairs that we have at a specific chromosome and start position which has a 1% chance of a crossover (or a morgan has a 100% chance of one crossover) per generation.

As these crossover chances are very different between men and women (women have a much higher crossover rate, they have about every 640k base pairs one crossover whereas men have it on average every 1.09 million base pairs).