What _is_ a Kosambi centimorgan?

David Mathog mathog at seqvax.caltech.edu
Wed Nov 4 12:31:00 EST 1992

In article <1d6e0tINNbvu at agate.berkeley.edu>, nigel at pathos.berkeley.edu writes...
>I was just wading through this months Genetics and ran into a term I
>can't figure out.  As the subject line says: What is a Kosambi
>centimorgan?  How does it differ from a plain vanilla centimorgan?  Is
>there a difference? 
>ref: Stuber et al. Genetics 132(3):823-39, November 1992

This answer is copied straight out of a preprint of a paper by Piet Stam in
which he discusses his JOINMAP program. (Sorry, I don't have the final

"The map distance between two markers is defined as the mean number of 
recombination events, involving a given chromatid, in that region per 
meiosis.  This is usually expressed in centimorgans (cM).  A map distance 
of 100 cM corresponds to an average of one recombination event per gamete.  
The relation between map distance and recombination frequency (and vice 
versa) is expressed by a genetic mapping function (mf). Different mapping 
functions correspond to different assumed degrees of interference in 
crossing over.  Interference is the phenomenon that recombination events 
in adjacent regions of a chromosome are not independent; in a classic three 
point test it results in a number of double recombinants which differs from 
what is to be expected on the basis of independence.  The most commonly 
used mapping functions are Haldane's (Haldane, 1919) and Kosambi's
(Kosambi, 1944) mf." 

  Haldane, J.B.S 1919.  The combination of linkage values, and the 
  calculation of distance between linked factors. J. Genet. 8:299-309.

  Kosambi, D.D. 1944.  The estimation of map distance from recombination 
  values.  Ann. Eugen. 12:172-175.

Since you may not be able to get your hands on these look in a good
Genetics / Linkage book.  Try:

  Ott, J.  Analysis fo human genetic linkage

Hope this helps,

David Mathog
mathog at seqvax.caltech.edu
manager, sequence analysis facility, biology division, Caltech

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