Identical Twins Aren't Identical?

Now here's an interesting topic; one that I have wanted to write a post about and now have the lovely time to do it!

For those who don't already know, a report was published in the February issue of
the American Journal of Human Genetics (AJHG) concerning genetic differences between identical (i.e. monozygotic - arriving from one fertilized egg) twins. Weird. Identical twins are supposed to be identical through and through, right? Isn't that why they're called identical? Well, maybe not. I suppose that is pushing it a little.

Anyway, Carl Bruder, a geneticist from the University of Alabama at Birmingham, and colleagues compared the genomes of 19 sets of adult identical twins. What they found was that these people differed subtly from their twin in what's called, copy number variation (CNV) of their genes.

As Wikipedia states, CNV's refer to:

"differences in the number of copies of a particular gene present in the genome of an individual . . . Some people have deletions of some genes on only one chromosome while other people have multiple copies of some genes."

People typically have two copies of each gene within their genome, because they inherit one copy from each parent. Yet at various genome sites copy number variation occurs. One would expect that monzygotic twins would resemble each other in the amount of CNV's that occur in their genomes. Since the "identical" twins from the above study differed in genetic copy number variants, they aren't 100 % identical!

It's all very interesting. And I think such findings may trigger important implications to consider for further psychological research. In fact, I learned about this study through class discussion in a personality research class and am frankly surprised that it hasn't received as much attention as I thought it would through the various media outlets.

What's one of the major issues most everyone will learn about in any psychology course? Well how about the battle between nature and nurture? Everything's a nature/nurture problem. And the common way to decide the influence of nature on a psychological attribute is to conduct a twin study. The logic behind a twin study is that identical twins should be most related to each other, as compared to dizygotic twins, non-twin siblings and strangers. Therefore differences between monozygotic twins on, say, a personality variable (and we do occasionally find these types of differences) aught to be attributed to environmental influences. But if identical twins are not really genetically identical, then how do we know that a personality difference isn't genetic? Now the AJHG report does not completely destroy the picture created by twin study results. Monozygotic twins are still highly similar; again, the differences in CNV's are subtle. Yet the paper does add the implication that an observed personality difference cannot automatically be attributed to the environment.

Oh well, if you want to read more about it, there is a Scientific American article about it, as well you can visit the AJHG site are read the original article. Go check it out! Very cool stuff!