Why are there 23 pairs of chromosomes?

David Curtis dcurtis at hgmp.mrc.ac.uk
Mon Jul 10 05:11:54 EST 1995

Since I'm being encouraged to increase the intellectual content of
this group, I'll make the following contribution.

As pointed out elsewhere, the number of chromosomes has little
relation to the number of genes. All animals share common ancestry, so
the fact that nowadays they have different numbers of chromosomes
means that at some point in evolution the number changed from parent
to child through trisomies, fusions and other rearrangements. In fact,
these events have carried on occurring until very recently in
evolutionary terms, and presumably may continue to do so. Except that
for humans almost all trisomies are lethal and certainly compromise
the capacity to reproduce dramatically. Are all tetrasomies lethal? I
would expect so. If so, it would seem likely that humans will be
"stuck" with 23 chromosomes indefinitely. Would most people agree? How
about other species? Is there any remaining flexibility in their
chromosomal complement?

On a tangentially related point, we know that the genome size depends
largely on how much junk DNA is around, but can anybody explain _why_
the puffer fish has so little junk DNA? 

Dave Curtis (dcurtis at hgmp.mrc.ac.uk)
Institute of Psychiatry, London 

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