Why are there 23 pairs of chromosomes?

Robert Robbins rrobbins at GDB.ORG
Mon Jul 10 06:59:44 EST 1995



On Mon, 10 Jul 1995, David Curtis wrote:

> 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?

Humans are no more "stuck" at the moment than they ever have been.  Viable
chromosomal re-arrangements are rare at the level of individuals, but not
so rare at the level of evolutionary time.  A comparison of the karyotypes
of humans, gorillas, orangutans, and chimpanzees shows that some changes
have occurred during the time that these four species diverged.  Of
course, that's over perhaps 10 million years and humans have been "humans"
(in the sense of possessing written language, significant culture, etc.)
for less than 10,000 years.  

Indeed, the duration to date of "human" existence on the planet is so
short, compared with other biological processes, that it can barely be
characterized as anything other than a momentary aberration.  Some
momentary aberrations prove to be durable -- that's why we consider
archaeopteryx to be the first bird and not just a unique, funny-looking
lizard.


> 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? 

Because all statistical distributions have tails, is why.  If we assume
that the accumulation, or loss, of "junk" DNA has a random component, than
the distribution of junk DNA across different taxa will be governed, in
part, by some statistical distribution.  Now, suppose I throw 30 coins in
the air, then score the result in terms in the numbers of heads and tails
that appear.  The odds that they will all come up heads is vanishingly
small -- on the order of one out of a billion.  But, if I repeat my toss
several billion times, a few of them will almost come up all heads. 
Asking why those particular tosses came up all heads would have a lot in
common with asking why a puffer fish has more junk DNA than other critters.

It may be that there are some non-random processes associated with the
accumulation and distribution of junk DNA, and if that's the case, then
there might be an answer to the why question.  But we do have reason to
believe that at least some of the processes are governed by chance, and
given that, it's useful to remember that "all statistical distributions
have tails."

It's also useful to remember that every possible INDIVIDUAL outcome of my
30-coin toss has the same probability of occurring (one in a billion) as
every other one.  That is, suppose my 30 coins were all different - one
penny, one nickel, one pfennig, one mark, etc. - so that each toss could
be recorded as penny:heads, nickel:tails, pfennig:tails; mark:tails, etc. 
Scored that way, there are about one billion different, individual
outcomes, each one equally likely.  Of those billion outcomes, there is 
only one way to get all heads (where # = heads and - = tails):

    ##############################

but 30 different ways to get one tail and 29 heads:

    -#############################
    #-############################
    ##-###########################
    ###-##########################
    ####-#########################
    #####-########################
    ######-#######################
    #######-######################
    ########-#####################
    #########-####################
    ##########-###################
    ###########-##################
    ############-#################
    #############-################
    ##############-###############
    ###############-##############
    ################-#############
    #################-############
    ##################-###########
    ###################-##########
    ####################-#########
    #####################-########
    ######################-#######
    #######################-######
    ########################-#####
    #########################-####
    ##########################-###
    ###########################-##
    ############################-#
    #############################-

and 30 x 29 ways to get 28 heads and 2 tails, etc etc.  The "fact" that a
50:50 outcome of heads and tails is expected does not mean that any
PARTICULAR 50:50 outcome is especially likely (indeed, each one is just as
unlikely as all heads) -- it just means that there are so many different
ones that could happen.

Bottom line: one should not confuse the results of statistical processes
with the results of causal processes...






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