Until relatively recently, scientists believed identical twins were completely identical right on down to the DNA level. But this left some puzzles: Why, for example, would one twin develop a disease like Parkinson's while the other did not? It seemed odd that this could be explained by environmental exposures alone. And on a totally anecdotal and unscientific level, I've known identical twins with different personalities, and it also seems unlikely that those variations could be explained entirely by subtle differences in the way the twins were treated in the same family. 

Now researchers have found possible DNA factors that complicate the picture and offer some reasons for differences among twins. When humans receive chromosones, sometimes there are pieces of DNA missing, or mutations cause a few copies of the same piece of DNA. This is called copy number variation (CNV) and in a study of identical twins, researchers found differences in CNV between sets of twins. Now maybe this will explain how a good person can have an identical evil twin... Mwah ha ha.

Anyone notice differences in their twins?