Talk:Centimorgan

50 cM distance means...

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)

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)

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I get a different result using Excel

I get a different result using Excel. In cell C1 I put 0 (the chance a single gene will unlink from itself). Then I used the formula =(C1*0.99)+(1-C1)*0.01 to determine the chance that a gene 1 cM away will unlink ((the chance it was unlinked) * 0.99 + (the chance it was linked) * 0.01). Then I copied the formula 49 more times and in cell C51 I got the result 0.31791516. I don't know that much about statistics, but the discrepance could be because either:

1) Excel doesn't have infinite resolution. Meaning my number is wrong,

2) because I was doing 50 descrete checks, not checks on a continuum. Meaning my number is correct or,

3) my error somewhere else

I'd be happy to be corrected. --Djfeldman 17:19, 12 July 2006 (UTC)

Hey, this is five years later. My understanding is that a centimorgan corresponds to a crossing-over probability of 0.0099006633 -- very close to 1%, but not precisely 1%: to get a crossing-over probability of 1%, you need about 1.0101354 centimorgans, and 50 * 1.0101354 centimorgans correspond to a .317915 probability of unlinking.

I doubt the difference between the two definitions is of practical relevance (particularly since any actual work is using genomic data these days), but from a mathematical point of view, the "one morgan corresponds to the length of chromosome that, on average, contains one cross-over per generation" definition is much nicer.

89.204.137.128 (talk) 09:35, 21 July 2011 (UTC)

Incorrect definition

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)

Use of parent unit

"The size of the canine genome was initially estimated from maximum likelihood predictions to be 27 morgans in genetic distance." PLoS Genetics 1(5), 2005. No shortage of other examples...

"A 50 cM distance means that the genes will reassort 50% of the time, which makes them unlinked. "

This is incorrect. Two genes located 1 cM apart will by definition result in an observed recombination frequency of 1%. Over small intervals, x cM is almost identical to a recombination frequency of x%, but for two genes located wider apart, the observed recombination frequency will be lower than the genetic distance in cM, due to double crossing-over events. Two approximation functions are commonly used to adjust for this, the Kosambi and the Haldane functions. So: recombination frequency is NOT identical to genetic distance in cM. AND, all human chromosomes except the Y chromosome are larger than 50 cM.

Mistakes in centimorgan page

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 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)

There is repetition in the Centimorgan== Relation to physical distance== section. I propose cutting the first paragraph and editing the first sentence of the second paragraph to include some of that information. Any objections? B82mo (talk) 12:06, 20 March 2017 (UTC)

References

Meaningless Definition

"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)

Bad Citation

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)

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Possible example to illustrate concept for biologist

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)

Chart needed

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