However, out of the 33 pairs of twins studied, researchers were only able to distinguish between two pairs of identical twins. Richards points out that the results could be more accurate if they decide to target more mutations in their study. "We assessed only about 500,000 genetic variants. However, one can easily genotype up to 1.5 million genetic variants, which would increase the probability of identifying the correct twin pair by approximately 3-fold."
Here's the rub: the technique for distinguishing between identical twins' DNA is not ready to be used in court, says Greg Hampikian, a professor of biology and criminal justice at Boise State University. "No one has done it in a forensic case, and it will likely be a while before it reaches general consensus and will be acceptable under the Daubert Standard in a U.S. court," he told Popular Science. According to this standard, scientific expertise is acceptable only if the technique it relies on has been tested, subjected to peer review, has a known error rate and is widely accepted in the scientific community.
In the French press, experts commenting on the Marseille twin case were quoted saying that the DNA technology required would be too costly: hundreds of thousand of euros, even millions were some of the numbers mentioned. Hampikian and Richards both disagree, saying it would be in the tens of thousands of dollars. In these cases, labs can't rely on standard FBI DNA tests, because the regions those tests evaluate would be identical for monozygotic twins. They would need to do a whole genome sequencing, says Hampikian, which now costs about $10,000. Hampikian adds that for better results, this would have to be combined with a study of methylation, chemical marks on the DNA that change with the environment (after birth, twins are exposed to different environmental conditions). However, there are no guarantees that differences will be found.
Hampikian explains that one problem is that you would have to study a great number of different tissues to say for sure that you could tell the twins apart, because the mutations are so rare.
Michael Baird, the laboratory director of the DNA Diagnostics Center (DDC) in Ohio told Popular Science that in the case of the identical twins he recently studied, a mutation could be found in a cheek sample, while the DNA from blood and fingernail samples was identical between brothers. "There are mutations but it is hard to determine where they occur," he says. Furthermore, Baird adds that a whole genome sequencing requires a larger quantity of DNA than is usually found on a crime scene.
To be sure they are convicting the right man, investigators in Marseille will need to rely on other elements, such as alibis, confessions and criminal history. They could also look at fingerprints, which are different between monozygotic twins, one of the hypotheses being that fingerprints differ because twins occupy different positions in the womb and move differently.
If none of this is available, having a twin can be an easy way out for suspects. In 2009 in Berlin, identical twin brothers were released following a spectacular department-store burglary. Only DNA was found that night, and the German court could not conclusively say to which twin it belonged and, as a result, couldn't risk putting an innocent man in jail. "We are proud of the German legal system," the twins had declared at the time.
I have a set of twin kids. Yes, human kids, not goat kids although it's debatable at a restaurant. So-called identical. We THINK we know which is which because one was heavier at birth. But up till almost age 5, no one could tell them apart. Now they are in High School, and lead very different lives. They don't eat what the other eats. One is more athletic. While I wouldn't have been surprised if they couldn't be told apart with DNA at birth, I doubt that's true now. Apparently there was no other evidence, like hair, which is a great source of personal info. Does one have dandruff? What did the rapists breath smell like? Halitosis? Right or left handed? Was either of these exposed to something the other was not? Twin or not, they are not the same person and a professional detective worth their pay would never accept otherwise.
quasi44 is correct. There is a lot more in investigating criminal case than DNA. A good detective follows a number of investigative techniques to determine the perpetrator of the crime.
The French and Michigan case are extremely rare. Identical twins in its self are rare. And identical twins committing crimes, rarer still. I wouldn't worry about it.
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