chromosome fusions/fissions

Floyd Waddle fwaddle at CHI1.UNCFSU.EDU
Wed Nov 27 10:53:15 EST 1996

As described in the textbooks, Robertsonian fusions are produced by the 
replacement of the short arm of one acrocentric with the long arm of 
another acrocentric.  In centric fusions, breaks occur at the centromeres 
of two acrocentrics and the portions of each centromere which carry the 
long arms are fused together.

All genetics textbooks I have seen describe one or the other of these two 
types of fusion.  The idiograms for human and chimp, however, show an 
entirely different type of fusion.  The human second chromosome was 
produced by breakage at the tips of two short arms with the subsequent 
fusion of the short arms.  The result was a dicentric chromosome in which 
one of the centromeres became suppressed.  Amazingly, I have a textbook 
in which the text describes the fusion as a centric fusion while the 
accompanying figure clearly shows short arm fusion.

Thus, I have two questions:
  1.  Why did the author misidentify the type of fusion even though he 
could see that he was wrong just by reading his own book?
  2.  I have not seen idiograms of species other than those of humans and 
apes.  The karyotypes easily available to me do not show the detail 
necessary to distinguish one chromosome type fusion from another.  Thus, 
I have not seen proof of either Robersonian fusions or centric fusions.  
The textbooks do not give references that I can look up.  Is there any 
literature that demonstrates the reality of either Robersonian or centric 
fusion, or are they merely figments of the collective imagination of 
textbook writers?

Chromosome fisions are also described in the textbooks but the authors do 
not explain where the telemeres come from which are necessary to close 
off the raw ends of the fissioned chromosomes.  Have fissions actually 
been demonstrated or are the textbook writers postulating them merely 
because they can think of no other method of evolution for mammals with 
large chromosome numbers?

Floyd Waddle

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