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Do Identical Twins Have Different DNA?

05 Mar

twins-1

 

DNA profiling is considered the gold standard for individual identification. DNA-containing bodily fluids found at crime scenes can often be linked to the perpetrator with a high degree of accuracy, often measured in one per billions. It is highly individual and therefore highly accurate for identifying a given individual.

But since identical twins begin as the same fertilized egg, they have identical genetic material (DNA). After fertilization, the fertilized egg divides into two cells. To produce identical twins, these two cells separate and then each progresses forward to produce an individual. This results in two identical individuals with identical DNA. Or does it?

Twins egg:sperm

 

Standard DNA testing uses the concept of Short Tandem Repeats (STR’s). STR’s are simply short segments of DNA that repeat in certain areas of the very long DNA strand found in all of us. The number of these repeats in the various locations are what allow DNA profiling to distinguish individuals so accurately. This is a complex, though not really difficult to understand, technique which is discussed in great detail in two of my books: Forensics For Dummies and Howdunnit: Forensics.

DNA Profile

But scientists have known for years that the DNA of identical twins is not perfectly identical. It might or might not start out that way at that first cell division but for sure as the cells divide and the individual grows within the uterus, minor DNA changes can occur. These are on the level of the base pair sequences that make up the DNA chain.

Another DNA technique called Single Nucleotide Polymorphism (SNP) actually looks at each base in the DNA strand and uses this for comparison with another strand to determine if they came from the same individual. This is the direction that DNA testing is going but for now STR remains the method of choice.

Identical twins would look the same using STR analysis but a deeper analysis using SNP would reveal variations, thus allowing identification and separation of two identical twins. Let’s say, blood is left at a crime scene and that blood is matched to a particular individual. Let’s further say that this individual is an identical twin. STR DNA analysis would not distinguish between these two brothers, But if SNP is employed, the one who left the blood at the scene can be distinguished from his identical twin.

The recent French serial rape investigation involving identical twins Yohan and Elwin would be a case in point. Applying the SNP technique in this situation would likely solve the case.

Pretty cool stuff.

Howdunnit Forensics Cover

 

From HOWDUNNIT: FORENSICS:

SINGLE NUCLEOTIDE POLYMORPHISM

Single nucleotide polymorphism (SNP) is a new technique that will likely see increased use in the future. The major problem at present is that it is expensive. We saw that RFLP fragments were fairly long, a drawback that lessens their value in degraded or damaged samples (discussed later). This problem was circumvented by the discovery of STRs, which are very short fragments. But, what if the DNA examiner could use single nucleotide bases as the standard for matching? This would increase the discriminatory power of DNA even further. This is what SNP does.

Let’s say that two sequenced DNA strands looked like this:

CGATTACAGGATTA and CGATTACAAGATTA

If we searched for an “ATTA” STR repeat, these two strands would be indistinguishable

since both have two ATTA repeats. But, with single nucleotide analysis the strands differ by a single base: The ninth base in the first sequence is guanine (G), while it is adenine (A) in the second one. SNP can be used with restriction enzymes in the RFLP technique, or with PCR, where it can be easily automated. Theoretically, this will allow for discriminating two DNA samples based on a single nucleotide difference.

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11 responses to “Do Identical Twins Have Different DNA?

  1. Tom Combs

    March 5, 2014 at 8:17 am

    Neat indeed!
    Thanks for the info -

     
  2. Brenda

    March 5, 2014 at 5:59 pm

    Thanks for this! So glad they can finally differentiate!

    I’ve read of a couple of cases where the identical twinwho didn’t know he was one did time and one on the East Coast involving women twins. How many have you read of?

     
  3. LC Aisling

    March 5, 2014 at 6:56 pm

    There even rises question on that?

     
  4. Eve Paludan

    March 5, 2014 at 7:09 pm

    Fascinating! Shared the link on my Facebook page.

     
  5. sandy gardner

    March 5, 2014 at 10:24 pm

    Dr. Lyle,
    thanks so much for this! It’s a question– whether identical twins have identical DNA– I’ve been wondering about for awhile.
    sandy gardner
    sgardner2@hvc.rr.com

     
  6. Lynne Lederman, PhD

    March 6, 2014 at 9:08 am

    DNA can also be modified by environmental factors (epigenetic modifications) whereby small chemical groups may be added to or removed from the DNA. This may not be used forensically (yet) but if identical sibs were separated and raised in different environments, their DNA could have significantly different epigenetic modifications.

     
  7. Sharon Hopkins

    March 6, 2014 at 10:14 am

    Really cool stuff and great info. Thanks!!

     
  8. Pat Marinelli

    March 6, 2014 at 11:57 am

    Great post and wonderful information.

    As an identical twin, I wondered how long it would be before they could do this. As kids we were always encouraged about our identicalness, but as adults we started to notice things that were not the same. For instance, I need more meds, hers need to be cut in half. Knock her out for surgery and she can’t wake up, me they have trouble knocking out and I wake up too soon. We both have food allergies, but to different foods. There are other subtle differences but I can’t think of any more off the top of my head.

     
    • D.P. Lyle, MD

      March 6, 2014 at 12:05 pm

      We are indeed all different–regardless of genetics. Thanks for sharing this.

       
  9. David McGill

    September 26, 2014 at 6:18 pm

    Need to know the name of a chemical or poison that would lower the heart rate to the 25-35 range – unresponsive to attempts to raise it medically, but correctable by pacemaker, and also that would raise liver panel results dramatically (AST to 160, ALT to 122) declining by half in 30 hours and returning to normal thereafter. Kidney function results would also rise temporarily but not nearly as dramatically. For more info please contact me at davidmcgill1931@gmail.com Thank you

     
    • D.P. Lyle, MD

      September 27, 2014 at 6:16 am

      David—I don’t answer such questions on this blog and without certain information. This can be found on my website (www.dplylemd.com). Please supply the required info and resubmit your questions via regular e mail.

       

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