Kosambi centimorgans

Toby Bradshaw toby at carson.u.washington.edu
Thu Nov 5 12:03:53 EST 1992

In article <1992Nov3.183314.7159 at mcclb0.med.nyu.edu> deustachio at mcclb0.med.nyu.edu writes:
>Nigel Walker asked:
>> What is a Kosambi centimorgan?  How does it differ from a plain vanilla
>> centimorgan?  Is there a difference? 
>and Joe Felsenstein replied:
>> I think a Kosambi centimorgan is just a name for an ordinary centimorgan,
>> when that centimorgan is estimated from recombination fractions, using
>> the mapping function of Kosambi:
>>    Kosambi, D. D.  1944. The estimation of map distance from recombination
>>    values.  Annals of Eugenics  12: 172-175.
>> There are other mapping functions, and there is no real reason to append the
>> name of the mapping function to the name "centimorgan" as if what it
>> calculates is incommensurable with ordinary centimorgans.
>It's not quite that simple.  Mapping functions embody theories about inter-
>ference (interactions - positive or negative - among a given number of
>progeny chromosomes into a 'true' (contingent on the validity of your theory)
>map distance.  Different mapping functions make quite different assumptions
>in this respect and, especially for low density maps (i.e., recombination
>fractions greater than 10% between adjacent markers), yield appreciably
>different results.  At the same time, all the mapping functions I know of
>converge as the distance between adjacent markers gets small. 
>Peter D'Eustachio
>NYU Medical Center - Biochemistry
>peter at mcbcm2.med.nyu.edu

I'm guessing that the creator of the "Felsenstein mapping function"
(Felsenstein [1979] A mathematically tractable family of of genetic
mapping functions with different amounts of interference. Genetics
91:769-775) is pretty well aware of the theory and practice of
genetic mapping, and of the underlying assumptions.  I think Joe's
point is that all of the mapping functions give the "true" map
distance if their assumptions are met; proving that the assumptions
are (or are not) met is the problem.

Toby Bradshaw
Department of Biochemistry and College of Forest Resources
University of Washington, Seattle
toby at u.washington.edu

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