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$Title{Not so Identical Twins}
$Subject{twins identical monozygotic fraternal dizygotic fertilized egg eggs
sperm chromosome chromosomes gene genes pregnancy birth}
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Copyright (c) 1993  Tribune Media Services, Inc.


Not so Identical Twins


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QUESTION:  What happened when a women has twins and they do not look alike?
You can't tell that they were born together by looking at them.  I would very
much like to have this information, as I am a daily reader of your column.

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ANSWER:  The definition of twins is two individuals who developed during the
same pregnancy.  The definition applies whether they look alike or not.  There
are two types of twins, "identical" (monozygotic) or "fraternal" (dizygotic).
In births of identical twins, a single egg is fertilized by a single sperm.
Shortly after fertilization, as the egg begins to divide, it splits into two
separate parts.  Each one contains the exact set of chromosomes and genes, and
the babies when they are born are look-a-likes, or identical.  However,
fraternal twins result from two different eggs being fertilized by two
different sperm, but at the same time.  They are as alike or as different as
any brother and sister can be.  They do not have to be of the same sex, as
they are really two completely different individuals.  Yet they are still
twins.  Twins occur in about 1 out of every 90 births in the United States,
and in 7 out of each 10 births of twins they are fraternal twins.  Fraternal
twins seem to run in families, so if you are one of a pair of fraternal twins,
you have a greater possibility of being blessed in the same fashion when you
have children.

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The material contained here is "FOR INFORMATION ONLY" and should not replace
the counsel and advice of your personal physician.  Promptly consulting your
doctor is the best path to a quick and successful resolution of any medical
problem.