What is/are the mechanism(s) of chromosome fusion?

prfwaddle at HUGO.FSUFAY.EDU prfwaddle at HUGO.FSUFAY.EDU
Fri Jul 22 16:08:13 EST 1994

  Before about 1970, the only chromosomes that allowed for detailed 
cytological analysis came from salivary gland and gut cells of Dipteran
insects.  Otherwise, the staining methods then in use allowed observation
only of the gross structure of chromosomes.  One could see constrictions,
centromere position and arm lengths, but very little else.  Comparisons of
related species and among individuals within species showed that chromosome
fusions were common in nature, and it was assumed these were Robertsonian

  There now seem to be three distinct mechanisms for chromosome fusion
depicted in the textbooks.  All are initiated by breakage of two acrocentric
chromosomes in a germ line cell.  In classical Robertsonian translocation
(aka Robertsonian fusion, centric fusion), which seems to be the only 
mechanism the textbooks explain in any detail, one break separates the long
arm of the first acrocentric from its centromere.  Another break separates
the short arm from the second acrocentric.  The long arm of the first chromosomethen replaces the short arm of the second to form a single metacentric 
chromosome with the essential genes of the two original acrocentrics.

  The phrase "centric fusion" is often used as a synonym for Robertsonian
translocation.  Some recent authors, however, appear to be using the phrase
to mean that the centromere of the derivitive metacentric contains DNA from
both centromeres of the original acrocentrics.  That would truly be a centric
fusion and not a Robertsonian translocation.  But what research demonstrates such
a fusion?  These authors do not say.

  The third mechanism does not seem to be described in the textbooks but is
obvious in drawings where banding patterns of human chromosomes are compared
to those of ape chromosomes.  One of the human metacentric chromosomes is 
equivalent to two ape acrocentric chromosomes.  It is not a Robertsonian 
translocation, however, as was assumed before mammalian chromosome banding
techniques were developed.  Nor is it a centric fusion.  The breaks in the ancestral
acrocentrics were at the tips of each short arm.  The derivative
human metacentric is actually a dicentric chromosome with one centromere 
suppressed.  The suppressed centromere is in chromosome band q21.2.  This 
mechanism does not seem to have been given a name.  Let us here call it 
"short arms fusion".

  The innovations of the past 25 years should have allowed cytogeneticists
opportunity for detailed analysis of chromosome fusions.  Short arms fusion,
however, is the only mechanism of chromosome fusion that I am aware has 
been demonstrated, and then only for the human situation.  Is the human 
situation the norlmal way fusions occur or is it an exception?  Has
Robertsonian translocation actually been demonstrated, or has it been assumed
to occur simply because it is more plausible than short arms fusion? 
Robertsonian translocation should allow for attachment of the long arm of an
acrocentric to the tip of one of the arms of a metacentric.  Has anyone 
identified such a fusion?  Has anyone produced solid evidence for centric 

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