Epigenetics – VII. Identical Twins (IT)
Date Posted: February 4th, 2010
How identical are identical twins really are? Epigenetics
has – at least partly – answered a question that has puzzled
humanity for centuries. Why do IT strive to be the same early in
their life, then reach a fork in the road when they go in
different directions and begin to more and more differ?
One in every 250 births will result in IT. They start and
end their lives with the same genetic package, but as they grow,
differences in their environment will alter their appearance and
behavior.
If one member of an IT set committed a crime and, in error,
left samples (hair, blood, etc.) which can be used to determine
his DNA, it would still be impossible to determine the culprit,
their DNA being the same. However, closer inspection at the
molecular level will reveal significant differences, and thus
will help identify who of the two is guilty. You see, they may
be identical genetically, but not epigenetically. Some genes
might be active in one twin but not the other.
Why the difference in gene activity (or inactivity)?
Biochemical fine tuning of the genome will determine which genes
are expressed or silenced. This point has already been outlined
under Methylation. I have also discussed the impact of Histones.
The conclusion here is that IT are not really identical. They
start that way, but then diverge. But the time they reach old
age, they are really very different individuals.
One more important issue: IT are not fated to suffer from
the same genetically inherited disease(s). Dr. Arturas Petronis
M.D., PhD, head of the Krembil Family Epigenetics laboratory at
the University of Toronto provides us with an example. There is
a high occurrence of IT with Bipolar Disorder, but Dr. Petronis
explains that epigenetics may account for the 30 – 70% of cases
where only one twin has the illness. While IT share the same
genome, their epigenome differs. Moreover, whereas DNA variation
are permanent, epigenetics modifications are in a state of flux
and generally accumulate over time. This may elucidate another
mystery, namely why Bipolar Disorder tends to appear at ages 20 –
30 and 45 – 50. This is due to the major hormonal changes at
these ages which may in turn impact genes regulations … via
their epigenetic modifications.
The good news here is that epigenetic disorders can be
reversed making them inviting targets for new drugs.
Sources
1) Epigenetics
Science in School
How epigenetics shapes life
www.scienceinschool.org/2006/issue2/epigenetics/
2) Epigenetics
A new science peels away another layer of the genetic onion
by John McManamy
www.mcmanweb.com/epigenetics.html
