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Keith Robison robison at golgi.harvard.edu
Wed May 31 23:09:57 EST 1995

Max ben-Aaron (xeno357 at ix.netcom.com) wrote:
: Greetings and/or salutations:

: Is there anybody out there who can answer this question?:

: What is it about a chromosome that gives it its integrity?  In other
: words, If I (were able to) cut one or more genes out of one chromosome
: and graft them into another,  or if I swapped some genes from
: chromosome to chromosome to could these genes be functional? 

In general, the answer to your question is that chromosomes are
(for the most part) merely vessels to carry the genes, and
genes can "jump ship" to another chromosome.  The most notable
exceptions would be genes on the sex chromosomes.  In the 
case of mammals, if the male-determining gene is moved off the Y
then you can have mammals which are chromosomally female (XX) but
phenotypically male (this has been done experimentally in mice,
and occurs naturally in humans).  Because some genes necessary
for male fertility are on the Y, such XX males are sterile.

The other complication regarding sex chromosomes is the issue
of dosage compensation.  In species with chromosomally-determined
sex, the sex chromosomes carry many general-purpose genes (i.e.
not directly related to gender).  In order to maintain the
same expression of these genes in both genders (one of which
has 2 copies and the other only 1), either the homogametic (XX)
copies must be turned down or the heterogametic (XY) copy must
be turned up in strength (both are observed, depending on the organism).  
If the mechanism of dosage compensation does not depend on 
chromosomal location (again, in some systems it does and others
it doesn't), then the gene may be trapped on the sex chromosome
-- as if it is moved to a autosome it won't have the copy number
problem but will have the copy number solution, and hence may not
be expressed properly in one gender.  Conversely, a gene on
an autosome may be unable to function properly on a sex chromosome,
since it will be missing the dosage compensation signals.

The other complication worth mentioning is that some genes are
intertwined with their neighbors, and so it may not be possible
to move individual genes to another chromosome (and retain
function) but it would then be possible to move the entire
block of genes to another chromosome (unless, of course, the
block consists of all the genes on the chromosome!).

Keith Robison
Harvard University
Department of Cellular and Developmental Biology
Department of Genetics / HHMI

robison at mito.harvard.edu 

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