centiMorgans & physical distance: a question.

anthonyp at scripps.edu anthonyp at scripps.edu
Tue Aug 29 14:53:37 EST 1995


In article <41v8seINNpt0 at essex.UCHSC.edu>, <binstoct at essex.UCHSC.edu> 
writes:

>        I have accumulated approximately 40 articles wherein
> chromosome length is reported and wherein male/female length
> differences are reported in terms of centiMorgans. 
>        
> QUESTION: 
>      Is the Gelehrter and Collins quote accurate? In other words,
> although not precisely linear, does physical length tend to
> correspond to centiMorgan length?
> 

As  you correctly state, the CentiMorgan is derived from the frequency of 
recombination events per meiotic event.  It is generally true that the 
longer the physical distance between two loci, the more recombination 
there will be.  However, anything that affects recombination will change 
the "distance" in cM.  For example, in some animals, recombination in male 
meiosis is much lower than that in female.  Also, within one organism, 
there may be stretches of DNA that are relatively refractory to 
recombination, perhaps because of physical constraints (around 
centrimeres, eg.).  Thus, a small physical distance may be mapped as quite 
large in terms of cM.  And, recombination rates vary wildly from organism 
to organism, so the relationship between physical distance and cM is 
different for yeast than it is for human.

Suppose you want to measure the distance between San Francisco and San 
Diego and some points in between.  But, you have no way to measure 
physical distance.  Instead you notice that traffic accidents occur more 
or less randomely along highways.  So you measure the distance in terms of 
the frequency of traffic accidents, "cA."  This is a good insight, but 
other things besides distance affect frequency of trafic accidents.  With 
the benefit of being able to look at the physical distances, you would 
notice some problems with your map.  First, the whole thing would go to 
hell around LA, where you would grossly over-estimate the distances 
between points.  Other stretches would be much shorter on you cA map than 
they are on the physical map.  

I believe it was Sturtivant that had the insight that, if genes were 
arranged linearly (which was not known), and recombination was a random 
event along that line, then recombination frequency would be related to 
the distance and relative position of the genes.  This was sheer genius. 
As a result, he could explain why there were different recombination 
frequencies between genes and construct a map.  The fact that he could get 
a consistent map supported the hypothesis.  Now that we know more about 
how genes are physically arranged, cM is a bit of an antiquated unit.  It 
is still useful to get a rough idea of distance, but we all know it is 
affected by too many factors other than distance to be a good unit of 
distance.

-tony



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