Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!silver!chiaravi
From: chiaravi@silver.bacs.indiana.edu (Lucius Chiaraviglio)
Newsgroups: sci.bio
Subject: Re: How do twins work?
Summary: Comments
Message-ID: <3791@silver.bacs.indiana.edu>
Date: 21 Apr 89 02:20:24 GMT
References: <839@garcon.cso.uiuc.edu> <1390@rpi.edu> <8243@boulder.Colorado.EDU>
Reply-To: chiaravi@silver.UUCP (Lucius Chiaraviglio)
Organization: Department of Biology at Indiana University, Bloomington
Lines: 54

In article <8243@boulder.Colorado.EDU> pell@boulder.Colorado.EDU (Anthony
Pelletier) writes:
>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.
>>
>>Normal human cells are diploid (They contain twice the necessary amount of
>>genetic material).  Sex cells are monoploid (only the necessary amount of
>>genetic material).  But, sex cells have their genetic material separated
>>in such a way that they contain (follow this carefully) twice the amount of
>>half the genetic makeup.  (Read a book on the differences between meiosis
>>and mitosis if this is still unclear)

	That is only true in eggs which do not complete the equational (second
in most organisms) division of meiosis until after fertilization.

>Anyway, there was alot of misinformation in you posting.
>The term you want is "haploid."  Monoploid is not incorrect, since it means
>"1n."  But haploid is the term used for gametes such as the 1n gametes
>of a 2n organism (or 3n gametes of a 6n organism, for that matter).

	That isn't the way I have always heard it -- the way I have heard it,
if an organism is tetraploid and makes gametes by standard meiosis, you are
obliged to refer to the gametes as diploid, etc.

>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 possible.

	That is only true of some organisms.  In others, the polar bodies
become separate cells after each division of meiosis; these cells would most
likely not contain enough materials to form an embryo successfully even if
fertilized.

>                                                 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
have seen scanning electron micrographs of eggs with polar bodies attached,
and the polar bodies have < 1/64 of the volume of the egg.  The smallest
fraction of an egg (formed by mitotic division of the zygote followed by
separation of the resulting cells) that I have heard of being able to form a
viable embryo in mammals is 1/16 (and that only in some mammals), so a polar
body of the size shown in these electron micrographs would die, possibly after
a few divisions.

-- 
|  Lucius Chiaraviglio   |  ARPA:  chiaravi@silver.bacs.indiana.edu
BITNET:  chiaravi@IUBACS.BITNET (IUBACS hoses From: fields; INCLUDE RET ADDR)
ARPA-gatewayed BITNET:      chiaravi%IUBACS.BITNET@vm.cc.purdue.edu
Alt ARPA-gatewayed BITNET:  chiaravi%IUBACS.BITNET@cunyvm.cuny.edu