Talk:Centimorgan
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- 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)
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)
[edit] 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.
