Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!boulder!pell
From: pell@boulder.Colorado.EDU (Anthony Pelletier)
Newsgroups: sci.bio
Subject: Re: How do twins work?
Message-ID: <8272@boulder.Colorado.EDU>
Date: 21 Apr 89 15:53:16 GMT
References: <839@garcon.cso.uiuc.edu> <1390@rpi.edu> <8243@boulder.Colorado.EDU> <3791@silver.bacs.indiana.edu>
Sender: news@boulder.Colorado.EDU
Reply-To: pell@boulder.Colorado.EDU (Anthony Pelletier)
Organization: University of Colorado, Boulder
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In article <3791@silver.bacs.indiana.edu> chiaravi@silver.UUCP (Lucius Chiaraviglio) writes:
>In article I wrote:

>>In article <1390@rpi.edu> fargo@pawl.rpi.edu (Irwin M. Fargo) writes:
>>>>
>>>Pathway 2 is not only very improbable, but it's very impossible in humans.
>>>
>
AP:
>>Finally, since ALL FOUR gametic nucleii (the egg nucleus and 3 polar bodies)
>>remain in the ovum until fertilization, something like what tim diescribed
>>really is not outside of the realm of the possible.
>
LC:
>	That is only true of some organisms.  In others, the polar bodies
>
>>                                                 Very occasionally, polar
>>bodies do get fertilized.  If the polar body that separated from the
>>egg nucleus at meiosis II was fertilized, as well as the egg nucleus,
>>the resulting fraternal twins would be exactly what tim suggested--that
>>is, they would have one set (the maternal) of chromosomes that was identical.
>
>	Not likely, unless an aberrantly large polar body had been formed.  I
>
(expands on why it is not likely)
>-- 
>|  Lucius Chiaraviglio   |  ARPA:  chiaravi@silver.bacs.indiana.edu

 Lucius,

I think you are being a bit unfair in your reading of what I wrote.
You point out, for example, that the possible scenario of a polar-body
geting fertilized is "not likely."  I believe
the term I used in my posting was "not outside the realm of the possible."
Does that really sound to you that I was suggesting it was likely? 
 
As for the development of the embryo, the fellow expressly requested we keep it
to humans, not other animal's.  What I described was essentially true for mice
and probably humans (it is a bit hard to do the experiments on humans).
The situation is very different for flies.
Basically, as you say, meiosis II is delayed until fertilization.  Actually,
it is arrested at metaphase II.  So, I oversimplified when I said that all four
nucleii were present--all the material is there, they are arrested just prior to
separation.  In mammals, the polar-bodies are not extruded until after
fertilization (hence the name).  As you said, it is not very likely, but
"it is not outside the realm of the possible" and "very occasionally"
(god, how many qualifiers do you want?!) a polar body can get fertilized.
Accurding to the information available to me, if this happens, it happens
BEFORE extrusion.  It is therefore possible (read: "not very likely" if you
prefer) for two nucleii to get fertilized and it is not unheard of (read:
even less likely) for both to develop in mice.
The reason I brought this up was to contradict the "very impossible" phrase
used by Ethan/Irwin.  It is common for beginers in biology to think we really
know what is possible and what is not. This is a habit that needs to be broken.


-tony