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Archiver > GENEALOGY-DNA > 2007-10 > 1191792699-01


From: Jim T <>
Subject: Re: [DNA] Why chromosomes ?
Date: Sun, 7 Oct 2007 14:31:39 -0700 (PDT)
In-Reply-To: <mailman.51923.1191774960.9735.genealogy-dna@rootsweb.com>


From: "Ron Scott" <>
> On Sun, 7 Oct 2007 07:55:05 -0700 (PDT), Jim writes:
> "Somewhere in the very distant past, we had an
> ancestor with one chromosome. At some point, that
> chromosome was duplicated, and we had an ancestor
> with two chromosomes. Organisms with two chromosomes
> were able to evolve faster, and out competed
> organisms with one chromosome."

> According to this logic, the potato, which has 48
> chromosomes, should be as evolved or more so than
> a human being. My suggestion was rather that it is
> the *number* of 22 autosomes that is significant,
> not who or what has the most in his/her package.
>
> Ron Scott

Ron,

Do you have any evidens for your suggestion that 22 pairs of
autosomes is more significant than, for example 21 pairs of
autosomes, or 23 pairs of autosomes?

I don't know what you mean when you say that a human being is
more evolved than a potato. It sounds like a metaphysical value
judgment. The fact is that potatoes do many things that human
organisms cannot do. For example, they can extract energy from
sunlight and nutrients from soil. Those processes require large
and complicated networks of genes which human beings lack. In
general, plants have more genes than animals. There are very
good reasons for this which I don't want to take the time to
discuss at the moment. I will just mention a couple of points:

- The common garden potato has 24 pairs of chromosomes, for a
total of 48 chromosomes. Human beings have 23 pairs of
chromosomes, for a total of 46 chromosomes - not very different
from the potato.

- The common garden potato is an autotetraploid, a kind of
polyploid. That means that a recent ancestor of the potato had
12 pairs of chromosomes (24 total), and through a mistake in
meiosis, it was doubled to 24 pairs of chromosomes (48 total).
(I'm oversimplifying this a bit, but there are many detailed
explanations on the web if you want to know more.) Many
commercially important crops are polyploids, including wheat,
maize, cotton, cabbage, tobacco, peanut, strawberry, and sugar
cane. The fact that polyploids have extra chromosomes is one of
the reasons why new varieties can be bred relatively quickly.
Plant breeding is human-directed evolution, and if polyploidy
speeds up human-directed evolution, it should also speed up
natural evolution.

Jim Turner



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