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http://www.scientificamerican.com/article.cfm?id=identical-twins-genes-are-not-identical

According to this article some identical twins show differences with respect to their copy number variants. How is this possible? If mitosis of an egg can lead to this difference, why can't mitosis of any cell in our body?

If we assume that identical twins are exactly identical, then if we make a clone of a twin will all three be exactly identical?

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Errors in division occur all the time and can show up in any dividing cell; this is, of course, important for cancer biology. If one of my cells replicates oddly right now it likely won't matter since it's only one out of trillions, but if that happened at a very early age in development it could be present in many if not all of my cells. Identical twins indeed come from the same egg-sperm fusion event but they develop separately from a very early state. It's trivial to imagine a minor replication error in DNA at an early cell stage that results in different genotypes between so-called identical twins. Besides, we've known for years that identical twins can have major epigenetic differences, so this isn't particularly ground-breaking.

If we assume that identical twins are exactly identical, then if we make a clone of a twin, will all three be exactly identical?

Depends on your definition of "clone" but how identical is identical? If you mean exact, then probably not; at the very least they will have different numbers of cells, for example, not to even mention CNV or epigenetics. If you want some SciFi cloning thing then, well, join the club.

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If I make a plant out of a single cell, will it be more appropriate to say that the new plant and the old plant are exactly identical genetically or to say that they are more like "twins" ? –  biogirl Sep 23 '13 at 17:38
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In theory, they should be more like "twins" but in practice you could say identical. For all intents and purposes there's no difference. –  Amory Sep 23 '13 at 17:42
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Identical twins do start out identical, but their DNA will quickly diverge. Not in a dramatic way, but in point mutations, at random locations, in different cells. Think of how there is a certain error rate associated with DNA replication - so even from that very first cell, when it divides to form a second cell, there is a chance there may be mutations in the DNA copy. It is highly unlikely that both twins will make the same mistake in copying the DNA in exactly the same location. Now repeat that idea millions and millions of times, as they develop, forming all the cells they need to form their bodies, and throughout their lives. Mutations are random, and they accumulate.

I made a video about this topic - Are Identical Twins Really Identical?

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@ biogirl: Why clone? There are quite a number of monozygotic quadruplets. One zygote giving rise to four identical beings. –  Ram Manohar M Sep 25 '13 at 6:47
    
The clarification that I require is whether the identical twins are exactly similar or are they mirror images? That is if there is hair whorl on the left side in one, whether it will be on the right side in the other. Functionally also if one is right handed will the other be left handed etc. –  Ram Manohar M Sep 25 '13 at 6:55
    
@khhsocratica Loved the video ! You guys are doing a wonderful job :) –  biogirl Oct 1 '13 at 18:38
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