Talk:Centimorgan

message: :A 50 cM distance means that the genes will reassort 50% of the time, which makes them unlinked.
This doesn't sound right to me. If two genes are 50 cM apart, the probability that they are exchanged is the sum of the odd numbered points in a Poisson density with λ=0.5, which is 0.316. -phma 02:14, 23 December 2005 (UTC)
replies: text: A cM is a map unit, 50 map units = unlinked.  Sounds right according to my notes, but I'll look around when I can get to my books again.  -- stillnotelf has a talk page 03:04, 23 December 2005 (UTC)
thread_title: 50 cM distance means...

message: The page states: "a centimorgan ... is defined as the 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."
I believe a correct definition would be something like: "a centimorgan ... is defined as the distance between chromosome positions (also termed, loci or markers) for which the expected average number of intervening chromosomal crossovers in a single generation is 0.02, and the frequency of recombinant meiotic products generated is 0.01."
This definition is in line with the Genetic linkage page:
"This distance is expressed in terms of a genetic map unit (m.u.), or a centimorgan and is defined as the distance between genes for which one product of meiosis in 100 is recombinant. A recombinant frequency (RF) of 1% is equivalent to 1 m.u."
For example, check out p. 162 of the textbook by Hartl & Ruvolo.  At the bottom of the 1st column they define:
map distance in cM = (1/2) × average number of crossovers in the region per meiotic cell × 100
C. elegans is a good example of that.  Every C. elegans chromosome undergoes exactly 1 crossover event during meiosis.  And every chromosome has a genetic map length of 50 cM.  
--Rbazev (talk) 15:05, 30 March 2016 (UTC)
thread_title:  Incorrect definition 

message: The centimorgan page makes two mistakes.  First, it attributes the name to Sturtevant.  In fact, the original unit was the "morgan", which was defined in 1919 by JBS Haldane J.B.S. Haldane. 1919. The combination of linkage values, and the calculation of distances between the loci of linked factors. Journal of Genetics, 8: 299-309. Second (and contrary to much modern usage), the morgan is not defined in terms of the recombination rate.  One morgan is the length of chromosome within which (on average) one cross-over occurs per meiosis.  A centimorgan is 1/100 of a morgan.  At this distance, the recombination rate is also approximately 1/100, but this is only an approximation: it is not the definition.
I propose that the "centimorgan" page be edited to reflect all this.  Any objections?
Alanrrogers (talk) 20:23, 14 November 2009 (UTC)
thread_title:  Mistakes in centimorgan page 

message: "It is defined as that distance between chromosome positions... ".  Distance is measured in metres.  Is this what is intended in this definition ?  If not, what units of distance are intended ?  Andrew Smith.  — Preceding unsigned comment added by 82.32.50.178 (talk) 09:40, 12 February 2013 (UTC)
thread_title:  Meaningless Definition 

message: Citation #3 is just a link to the webpage of some botany department. It doesn't mention anything about a Poisson distribution, let alone provide a justifying model.  — Preceding unsigned comment added by 128.250.5.247 (talk) 00:19, 6 December 2013 (UTC)
thread_title:  Bad Citation 

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thread_title:  External links modified 

message: I hope the editors most invested in this page will weigh in on my suggestion of an example in terms of how cross-over might "look" in terms of individual gametes. 
I think it would be helpful for biologists reading this article to have an example in the general introduction that relates the Centimorgan unit directly to the process of meiosis. I suggest a calculation estimating how "recombined" the genome of a gamete is likely to be. I know that there are a lot of things to consider, but some statement like "because of recombination during meiosis, each human gamete contains about 60 "grandparental" segments distributed across its 23 chromosomes." 
It seems to me 60 is a reasonable approximation for how "recombined" or "chunked-up" the genome of each human gamete is. Using 1 Mb per Centimorgan and 0.01 occurrences per generation implies a break every 100 Mb, making about 30 breaks in the genome resulting in about 60 chunks of the original 23 chromosomes. The number 60 also seems to work pretty well considering that: "... the number of crossovers (COs) is tightly constrained. At the lower limit, there is at least one CO per pair of homologous chromosomes per meiosis, and this “obligate” CO is essential for the balanced distribution of chromosomes in the gametes. At the upper end, the number of COs is relatively low in most eukaryotes, typically in the range of one to four per pair of chromosomes."  (Wayne Crismani and Raphaël Mercier, Cell Cycle 11:19, 3527-3528; October 1, 2012). If each chromosome is broken at least once, the lower limit is 46 chunks. Break chromos 1 and 2 in 3 places and the result is 52, etc. 
ThanksJohnfravolda (talk) 23:40, 30 June 2017 (UTC)
thread_title:  Possible example to illustrate concept for biologist 

message: Surely this article needs a definitive / best possible chart linking centimorgans to degrees of cousinship???    Icairns 2 (talk) 17:05, 2 August 2021 (UTC)
thread_title: Chart needed