Scientific American, March 2012 -- "What Makes Each Brain Unique?", Fred Gage & Alysson Muotri
Gage and Muotri describe mobile elements, or "jumping genes," short genetic segments that are able to be cut/copied and reinserted into DNA. This can have several possible effects on the genetic code, introducing amino acid substitutions, repeats, or stops in coding regions, changing a given protein, or altering noncoding regulatory region, changing how much a given protein is expressed.
Mobile elements are active only in reproducing cells, meaning their effects are often seen in the gonads, but they're also particularly active in the brain at sites of neurogenesis, such as the hippocampus. Gage and Muotri (or the editors) play up this element to argue that these genetic differences are what contribute brain differences, even in identical twins. I think this is a bit of an overstatement on their part, however. The environment, or the unique experiences a person has, has direct and continuing effects on the development of the brain. As such, mobile elements can help explain how this process occurs, and why the effects of even the same environment can differ among individuals (especially given the random nature of mobile elements), but it's only a part of the picture.
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