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nafex - RE: [NAFEX] candid chimera

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  • From: "Stephen Sadler" <Docshiva@Docshiva.org>
  • To: "'North American Fruit Explorers'" <nafex@lists.ibiblio.org>
  • Subject: RE: [NAFEX] candid chimera
  • Date: Thu, 17 Feb 2005 13:00:41 -0800

Chimeras occur in mammals when the mother produces two fetuses which fuse in
utero. This happens with some frequency in cattle, but I know of no cases
in humans, though it's theoretically possible.

There is another condition where cells arising from one zygote can have
slightly different genetics. This is called mosaicism. In mosaicism, the
gene expressions of some cells is suppressed. For example, in female
mammals, all but one X chromosome is inactivated early in development. Half
of those X chromosomes inactivated will have been paternally inherited, and
half maternally inherited - so there is some genetic difference, or a type
of mosaicism, present in female mammals. Different types of mosaicism are
fairly common in animals.

One type suppresses some of genes responsible for eye color in some cells;
so some cells that develop into an iris will have switched-on instructions
for blue eyes, and others will have active instructions for making brown
eyes. This is the same type of mechanism which causes calico in cats.

As far as how chimeras occur in plants, please look at Lee's explanation.
It doesn't happen in one individual organism - it requires a fusion of two.


Mosaicism arises from a single seed or zygote.

Stephen Sadler, Ph.D.
USDA 9, AHS heat zone 8,
Sacramento CA - Mediterranean climate

>

I wasn't familiar with this term used in a botanical sense,
so I looked it up and found:

"an individual, organ, or part consisting of tissues of
diverse genetic constitution"

Would a person with one blue eye and one brown eye be
considered a 'chimera' in this sense of the term?

*Why* would this happen within an individual organism, such
as one apple tree or one tomato plant?

Pat







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