Why are there 23 pairs of chromosomes?

[Graham Dellaire]

You wrote

-   roach at u.washington.edu (Jared Roach) writes:
-  	I thought I might add a couple comments.
-  
-  1) The evidence is overwhelming that there have been several independent
-  tetraploidizations during vertebrate evolution.  Most of the interesting
-  papers (with the hardest data) come from the Salmonid geneticists; look in
-  Medline for these.  Also, up my own alley, the vertebrate immune system
-  clearly arose in its present form following a tetraploidization.  It is
-  the simplest and perhaps only reasonable explanation for the parallel
-  genomic structure of the heavy and light immunoglobulin chain genes and
-  the alpha beta gamma and delta T-cell receptor genes.
-  	Thus not all tetraploidizations are lethal.  Why?  I dont' know.
-  Neither does anyone else.
-


I thought that too at first... but this is just a theory and the fact that we can't
forseeably resolve tetraploidizations easily with the recipient of the new
"scrambled" genome being able to mysteriously procreate.

The other extreme is to say that no genome tetraploidizations occured and that
all gene families arose by gene conversion (say for the four HOX clusters), perhaps
primed by all those repeats in the "junk" DNA as you termed it( LIne-1 and Alu's etc); and then
local amplification into clusters of similar genes at one chromosomal site via "slip pairing" during 
replication or by unequal crossover (to produce individual HOX genes in each cluster).

An interesting compromise.... is proposed by Richard Gordon and C. Cristofe Martin 
in " Differentiatin Trees, a Junk DNA molecular clock, and the evolution of neoteny in salamanders"
J. or Evolutionary Biology (1995)

I don't have the full ref. with me... sorry.

later,

Graham


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