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>
> I have accumulated approximately 40 articles wherein
> chromosome length is reported and wherein male/female length
> differences are reported in terms of centiMorgans.
> 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
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