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Identical Twins Not Identical After All
Hugh Pickens writes "Contrary to previous beliefs, identical twins are not genetically identical. Researchers studied 19 pairs of monozygotic, or identical, twins and found differences in copy number variation in DNA which occurs when a set of coding letters in DNA are missing, or when extra copies of segments of DNA are produced. In most cases, variation in the number of copies likely has no impact on health or development but in others, it may be one factor in the likelihood of developing a disease (pdf). "Those differences may point the way to better understanding of genetic diseases when we study so-called discordant monozygotic twins....a pair of twins where one twin has a disorder and the other does not," says Carl Bruder, Ph.D. "If twin A develops Parkinson's and twin B does not, the region of their genome where they show differences is a target for further investigation to discover the basic genetic underpinnings of the disease.""
Blindingly obvious? It requires a tremendous ability to detect genetic differences when the difference is a nothing more than the number of copies of a few base pairs in an entire genome (~3,000,000,000 base pairs). We still don't know whether there are any epigenetic differences, or what those might be, because that is even more difficult to measure.
While it may seem obvious to the uneducated that twins are "different", there is a lot of research that shows high correlation, even when the twins are raised apart, so identifying the cause(s) of the differences and whether those are "nature" or "nurture" is still of value. Even within a family, the differences may be simply something like feeding order, where the earlier fed may get different (not necessarily better or worse) nutrition or bonding experience than the later fed, rather than, necessarily, a genetic difference.
When it is copy numbers, or very small polymorphisms, and there is some somatic variation, we can use the data to more closely identify which genetic values are associated with the variation.
I remember that during medical school (maybe 7 or 8 years ago) we were told that while identical twins have extremely similar DNA to each other, that DNA is not 100% identical. Maybe 98 or 99% (more than any other two individuals on this planet), but not 100%. So while this is an interesting research, it hardly qualifies as "news"...
I'll tell you what I first thought when I read this:
I read in, I think it was a Steven Pinker book, about studies done on identical twins using the big five personality traits. What he said was that on the big five, identical twins raised together were roughly 50% similar, and identical twins raised apart were... roughly 50% similar. So when it comes to nature/nurture on the big five, you get 50% genetics, maybe 1-2% environment, the rest comes from ?????.
As you say, those numbers are probably based on the assumption of the genetic code being exactly the same, so those numbers can't represent reality as we know it now. But I'll be interested to see how the numbers fall out after the variance in twins' sequences is factored in. I would guess that it's the explanation for some of the ????? above, but how much?