marker number??

Mike Miller mbmiller at SIRRONALD.WUSTL.EDU
Tue Jul 11 14:40:02 EST 1995

On 29 Jun 1995, Timothy R Jackson wrote:

> ... also, my advisor says that, theoretically, one chromosome arm 
> should equal 1 Morgan.  is this valid?

For the diploid organism (e.g., human), each meiosis results in four
chromatids.  Suppose we select a random chromatid from a random meiosis
and count the number of crossovers it experienced between two points A and
B.  This number is a random variable and its expected value is equal to
the map distance in Morgans between A and B.  Thus a region of one Morgan
in length should have an average of one crossover per chromatid.  In
reality we are not able to count the crossovers.  We estimate their number
using recombinations.  An observed recombination corresponds to some odd
number of unobserved crossovers.  We use some mapping function (there are
several to choose from) to relate map distance to recombination fraction. 
This means that the operational definition of the Morgan unit depends on
the (fairly arbitrary) choice of map function.

However, I know of no mapping function that restricts chromosome length 
to 1 Morgan.  So I think your professor was mistaken.  Please tell us if 
he still doesn't think so.


Michael B. Miller, M.S., Ph.D.
Department of Psychiatry (Box 8134)
Washington University School of Medicine
4940 Children's Place,  St. Louis, MO 63110
office phone: (314) 362-9428    FAX: (314) 362-9420
WWW Homepage:

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