DNA testing has revolutionized murder investigations and paternity issues to such an extent that to get a mystery going, TV dramas often need to come up with weird situations where the tests won’t work. For example, CSI has had to resort to rare genetic conditions like chimerism to add a little spice to a story. And of course, there are always identical twins.
Identical twins start off from the same fertilized egg and so start out sharing exactly the same DNA. This means that standard sorts of DNA tests are useless in telling them apart. Which also means that crime shows can always add a bit of mystery by having an identical twin commit a crime or father a child.
Unlike chimeras though, identical twins are actually pretty common in real life. The latest estimates are that six out of every thousand people are identical twins. This means that there are a lot of children being fathered by identical twins and that these twins are involved in a good number of crimes too. And until recently, none could be identified from just their DNA.
This has all changed in a new study where scientists were able to reliably use DNA to tell two identical twins from each other. The scientists had to include the child of one of the twins in order to conclusively tell them apart but otherwise, the procedure took only a few weeks and a few thousand dollars. Probably too time consuming and expensive for every paternity or criminal case involving identical twins, but it might be worth it to catch a murderer.
Scientists have known for a while that in theory, the right kind of DNA test would be able to tell identical twins apart. This is because although identical twins start out with the exact same DNA, by the time they are born their DNA is just a bit different. The differences are so small though that a DNA test needs to be very sensitive to find them.
These differences (or mutations) are not specific to identical twins; we all build them up as we grow and develop. All of us, including identical twins, start out as a single cell. This cell and many of the resulting cells divide and divide again until there are the trillions of cells that make up each one of us. Each time that a cell divides, it needs to copy its DNA. And this is where the differences can arise.
Our cells are incredibly good at faithfully copying their DNA but they aren’t perfect. The occasional mistake slips through. Current estimates are that we each have less than a hundred differences in our DNA. Sounds significant but it isn’t a lot when we have over six billion spots to look at!
Using the mutations that arise after identical twins have split is complicated for a couple of different reasons. First, not every cell will have the same set of differences. If a mutation arises later in development, only the cells that came from that original cell will have that mutation. This is why we are all mosaics of cells with slightly different DNA.
This issue can be overcome for testing by making sure to use the right cells for what is being tested. If blood is left behind at the scene of a crime, make sure to test blood samples. And if it is a paternity case, the best tissue to use is probably sperm.
The other big issue is that our ability to read DNA isn’t perfect yet. The few mistakes we make aren’t usually a big deal but when we are looking at so few differences, they can begin to matter.
To get around this problem, the authors of the study included the DNA of one of the twin’s children in the test as well. By doing this they were able to identify unique changes that one twin had that he passed down to his child. Since the changes were in both parent and child but not in the parent’s identical twin, these changes are unlikely to be due to some glitch in the DNA sequencing. The authors managed to find five of these differences. This result allowed the scientists to establish paternity and could also have been used in a criminal case.
The bottom line is we now have the technology to figure out which identical twin fathered a child or committed a crime. Now having said this, what we don’t yet know is whether such a tricky procedure can routinely be done by crime labs. And whether or not we as a society care enough to pay the money to solve these cases. But one thing is for sure, in the near future TV dramas won’t be able to use identical twins to spice up their stories.