Genetic Sleuthing, Or How To Catch The Right Identical Twin Criminal

The police in Marseille France are struggling to solve a sexual assault case. They have solid video evidence and have even matched DNA from the crime scene with two suspects but they still can’t figure it out. See, the problem is that the suspects are identical twins.

Identical twins look pretty similar so unless the police get lucky like they did in Boston, video evidence can’t usually be used to tell them apart. And identical twins share the same DNA so conventional DNA tests can’t be used either. The police are in a real pickle.

Except that they don’t have to be. As the police are aware, there are less conventional tests that can find the few differences between the DNA of identical twins.

None of our DNA is exactly the same as when we were a fertilized egg, floating towards our mother’s womb. Our life experiences change our DNA in many different ways.

For example, there are chemical marks on our DNA that help to control which genes should be turned on where and to what level. These epigenetic marks are added and removed over our life time in response to our specific set of experiences. This is true for all of us including identical twins. In fact, scientists have actually looked at these epigenetic markers in identical twins and have found them to be different.

Another way our DNA can change is through mutation. DNA is not as stable as you might think and each of our DNAs is building up small changes over our lifetime. Sometimes these mutations can lead to trouble (for example, almost all cases of cancer come from DNA mutations) but most are harmless.

These mutations are random events that can happen in a couple of different ways. Sometimes something in the environment like cigarette smoke or the ultraviolet light from the sun damages the DNA. Other times our cells make a mistake when they copy their DNA and that mistake is passed on to the next generation of cells.

In both cases, the mutations are going to be specific to a person. The odds are definitely against two different cells making a mistake in the same place in each of their DNAs. Same thing with ultraviolet light targeting the exact same two T’s in two different people.

So there are unique DNA differences between identical twins that scientists can use to tell them apart. Why aren’t the police using these differences to catch their criminal? Because the cost is too high.

The police are saying that it would cost one million euros to do the required testing. No matter how hard I try, I can’t seem to wrap my head around that price tag.

I asked around and right now it costs between $5000 and $10,000 U.S. dollars to sequence an entire genome here in California. I know things are more expensive in Europe but unless the euro has crashed big time in the last few weeks, I can’t get to a million euros.

They would need to sequence the DNA of both twins and the DNA from each of the six crime scenes. Let’s call that $100,000 U.S. Do they really need an extra $900,000 to analyze the data?

Now to be fair, getting this information isn’t that easy. The mutations I was talking about happen in different places in different cells. A person may get a DNA change in his skin cell different from one in his blood or cheek cell. So the police need to be very careful about which tissues they choose to test.

For example, since these are sexual assaults there may have been semen from the rapist at some of the crime scenes. Sperm cells have a pretty high rate of mutation because men are constantly making new sperm. In fact, these mutations are why the children of older men are at a higher risk for certain diseases. So sperm would be an ideal source for looking at the whole genome.

But sperm wouldn’t be as ideal for looking at epigenetic changes. Before DNA is packaged into sperm, most of the epigenetic markers are wiped clean. Not all of them (which is one way environmental effects can be passed from one generation to the next), but enough so that this may not be the best approach with sperm.

The epigenetic approach would be better for other tissues. It is more expensive but scientists need to look at less DNA so the price pretty much evens out in the end.

The bottom line is that there are tests available to distinguish between the twins and that the tests probably wouldn’t cost one million euros. Still, even if the tests are “only” 100,000 euros, that is still a lot of money. It is up to society and the government whether or not catching one (or two) criminals is worth this cost.