"Identical twins like these can finally be told apart at the genetic level for only a few thousand dollars.  Image courtesy of Wikimedia Commons: http://commons.wikimedia.org/wiki/File:Redhead_twins.jpg
Identical twins like these can finally be told apart at the genetic level for only a few thousand dollars. Image courtesy of Wikimedia Commons.

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.

A couple of the ways DNA can be changed.
A couple of the ways DNA can be changed.

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.

Genetic Sleuthing, Or How To Catch The Right Identical Twin Criminal 7 March,2013Dr. Barry Starr
  • Also some identical twins have opposite hair swirls or are opposite (left/right) handed. Even Sherlock Holmes without DNA testing could determine those differences in a photograph/video or determining the marks left on a body by the way the killer used the weapon. Cost is a lot less than 1Million Euros/Dollars.

    • Barry

      Freckles would be a great way to tell the difference too. Where freckles happen is pretty random so if identical twins are freckled, you might be able to distinguish them based on their specific pattern of freckles. You’d need a pretty good picture though.

      • Twin1

        lol this is so funny and interesting. I am an identical twin so i find this very interesting to read. My sister and I are both right handed, straight hair, same height and weight, both with freckles all over. There is only one very very small difference and no one would ever know unless they are told and this is how our parents told us apart. Its a birth mark. one on the bottom of my foot and hers on her chest right above her heart. Growing up as an identical twin was crazy. Crazy as in good, awesome in fact. The people you could trick, confuse, make them think they are going nuts. We had a lot of fun with it. Hard to get in trouble when no one can say for sure who did it.

  • Van

    Could they not do some old fashioned serology tests to distinguish between the two based on proteins? That would be a lot quicker and much less expensive.

    • Barry

      Good point. What I don’t know is what samples they actually have (for example, a hair or semen sample may not have the sort of information they need for this kind of test) nor do I know how much they have. I also don’t know how easy it is to tell identical twins apart with this sort of testing either. If anyone knows more about any of this I’d love to hear about it!

      • John Fiorentino

        Sure, I sent you a partial pdf you can read when you have the time. It’s too much to post here, but I will reference it for those who may wish to buy it.

        The Forensic Laboratory Handbook Procedures and Practice
        (eds.) A. Mozayani; C. Noziglia 2011
        ISBN: 978-1-60761-871-3
        Humana Press


        • Barry

          Thanks John. I gave it a quick read and what I saw was using serology to identify the sample (for example using AP to positively ID a sample as semen). Didn’t see anything about serology being used to distinguish two people. Doesn’t mean they aren’t used that way I just didn’t see it I. The PDF.

          • John Fiorentino

            Well, the pdf was only a partial section from the book, which I am not able to post and to which I provided info. for purchase or for those who are fortunate enough to have good libraries nearby.

            Remember we’re talking “forensic serology” or more correctly “forensic biology” and is used everyday as my first post indicated.

            You can distinguish identical twins with forensic serology due to the fact that identical twins have the same DNA profile, but completely different antibody profiles. They also have different fingerprints, etc. and other differences as mentioned by another poster.

            A DNA test, with the price tag mentioned here is 1. not needed and 2. far too expensive.

            DNA testing is what you would do to determine whether the twins were identical or fraternal. In that case serology would be the wrong choice of testing.

            I hope this helps.


          • John Fiorentino

            Just a few more remarks after reviewing this. 1st. I sent you the wrong (or partially wrong section from the pdf. sorry)

            There may be RARE instances where a DNA test such as mentioned here is the only hope of determining differences between “identical” twins.

            In other words where serology can’t be utilized but we do have some DNA evidence.


            Some respected (as in most) scientists in this field believe as thus:

            With identical twins, even if you sequenced their whole genome you wouldn’t find difference…they’re clones,” said Dr. Bob Gaensslen, a forensic scientist at Orchid Cellmark labs in Texas. “There are a few things in science that are cut and dried and this is one of them.”

            Dr. Bob Giles, a paternity testing expert, agrees. “There is simply no test that explains the difference between two identical twins,” he said.

            And regarding the tests alluded to here, I believe the percentile of success equates to only 35%, which in court just wouldn’t cut it.


          • Barry

            I spoke with a number of scientists here at Stanford and they are able to distinguish identical twins with current DNA sequencing technology. Now whether this would hold up in a court of law is a different story as there isn’t any legal precedent for this but the courts probably will allow it at some point. Also, identical twins start out as clones but do not remain so until adults. There are small DNA changes or mutations (especially in the semen) that happen over time that make them different enough for newer techniques to tell the difference between them. In other words, old school DNA testing could not tell them apart but the newer techniques can. The big question was whether it could be done in an affordable way or not.

          • John Fiorentin


            The scientists at Stanford no doubt can do as they claim, however, in the case you sight I think AGE WOULD be a factor, The mutations you allude to are age dependent and the best percentile confidence I’ve seen is about 35%.

            The norm for scientific purposes of affirmation is generally 95%.

            The courts won’t work with 35% that I can tell you from personal experience.

            If the scientists at Stanford wish to correct me, I’d be most interested in hearing about it.


    • Barry

      I did a quick search and can’t find any examples where the police used serological tests to distinguish identical twins in a crime investigation. It is a great idea in theory, I just wonder if it has ever been used before.

      • John Fiorentino

        It’s a lot more than “theory”

        Criminal Investigation (Third Edition)
        Ronald F. Becker

        Chapter 1
        Introduction to Criminal Investigation — Page 8.

        Forensic Serology (excerpt) – Posted under “Fair Use”

        “Consider that identical twins may have the same DNA profile but completely different antibody profiles, and you begin to see the future of forensic serology.”

        But you’ll have to BUY the book, and it isn’t cheap!

        BTW – The best “Google” is in your head… 😉


  • John Fiorentino

    Forensic serology has been used for quite some time to make these determinations.

    DNA may have the same profile, but the two individuals would have a completely different antibody profile.

    In fact in many laboratories DNA testing and serology are combined. The term used is “Forensic Biology.”

    In fact there are a multitude of serology test kits on the market for much less cost!


  • John Fiorentino

    A final suggestion for this forum – Provide an edit function and allow the ability to post “attachments” such as pics, pdf’s etc.



  • Chris MacMillan

    I am fasinated by this story, reading the story from a scientists point is interesting, from the point of a person who was born with genetic clone, a identical twin, I participated in many studiesin the 60’s, after growing up I realized it waslike a MENGELE STUDY, none the less, when DNA became big My parents took our baby clothes with spit and hair on them and only family knew which belonged to whom, the scientists, not once identified correctly who belonged to which clothing, I repeat not once

  • Patricia

    Serology is performed on serum, that is, the clear liquid that separates from blood when blood is drawn and left to sit at room temperature. Antibodies occur in the serum, and can be identified using known antigens. Also, the level of individual antibodies can be quantified. It would seem to me that serology would only be helpful in determining which twin committed a crime if enough blood could be collected from the crime scene to allow testing for antibodies.

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Dr. Barry Starr

Dr. Barry Starr (@geneticsboy) is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA and runs their Stanford at The Tech program. The program is part of an ongoing collaboration between the Stanford Department of Genetics and The Tech Museum of Innovation. Together these two partners created the Genetics: Technology with a Twist exhibition.

You can also see additional posts by Barry at KQED Science, and read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.

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