What if the DNA evidence left at a murder is from an identical twin? Can scientists tell which twin killed the person?
-A curious adult from California December 19, 2008 It seems like a police officer doesn't have a shot in this case. Identical twins after all are identical. They start out with the same DNA. But DNA isn't a constant thing. DNA itself builds up small differences over time. I've dealt with this possible way of telling identical twins apart in a previous answer, so I won't go into too much detail about this here. Another way to tell identical twins' DNA apart has to do with something called methylation. Methylation is a mark on DNA that tells a cell how often a gene should be read. These markings change as we age in ways dependent on our life experience. This means that identical twins have different DNA markings on their genes. Scientists have recently figured out how to quickly and easily figure out how much methylation there is at a bunch of genes all at once. Someday, perhaps in the not too distant future, scientists will be able to use this method to catch a murderer. Genes Can be On, Off, or in Between Genes are an important part of our DNA. Each gene has the instructions for making a protein and each protein has a specific job it does in the cell. Not all genes work in all cells though. For example, our skin doesn't need a protein that helps us see. So the gene responsible for this protein is off in our skin and on in our eyes. Genes can also be turned up or down. In other words, a gene that is on is not always on at the same level. Sort of like a dimmer switch on a light. What this means is that there are differences between how much genes are turned on between different kinds of cells. And between different people too. Including identical twins. Identical Twins Use Their Genes Differently Scientists have found that when identical twins are born, they use their genes pretty similarly. For example, the genes on in blood cells are pretty much at the same level between the two twins. The same is true for all of their different genes and cells. As the twins age, though, this pattern changes. Now some genes are on higher or lower in one twin compared to the other. These changes reflect the different life experiences of each twin. One way scientists can see these changes is by looking at methylation around genes. Methylation is really just how many methyl groups are stuck in or near a gene. And a methyl is just a small chemical group. In general, a gene that is on at a low level has more methylation than one on at a higher level. So two genes with the exact same set of DNA letters might have different levels of methylation in each identical twin. This difference can be used to tell one twin from the other. Recently scientists have come up with ways to quickly figure out how methylated lots of different genes are all at once. This might be a way to tell identical twins apart from DNA evidence. And so catch our murderer. So maybe scientists could look at the DNA left at a crime scene and match its methylation pattern to one twin or the other. This would require that the crime be recent since methylation patterns do change. The bottom line is that there are definitely differences between the DNA of identical twins. Finding these differences will be hard but it is getting easier all the time. Soon, we may be able to put an identical twin in prison with just DNA evidence.

Identical twins have slightly
different DNA. Especially
as they get older.

As identical twins get older,
their DNA methylation
patterns change.
Methyl groups are in green.