How a New DNA Test Can Distinguish Between Identical Twins 

By Sarah Zhang on at

Ever since police started using DNA tests, one particular loophole has captivated our imagination: how do you distinguish between identical twins who share DNA? But it turns out even identical twins have tiny differences in their DNA, and prosecutors in the US state of Massachusetts want to use a new test for identical twins in court for the very first time.

The suspect on trial is Dwayne McNair, reports Wired, who allegedly committed two rapes in 2004. Another man involved in the crimes has identified McNair, and a DNA test matches but, he has an identical twin.

Ordinarily, forensic tests look at only several DNA snippets that are highly variable among people. With identical twins, who develop from the same fertilised egg, this approach obviously doesn't work. Suffolk County prosecutors instead sought out a new DNA test developed by the company Eurofins Scientific, which uses ultra-deep, next-generation sequencing that is becoming popular in genetics research labs. Eurofins' test decodes the entire DNA sequence in a sample, looking for the handful mutations that randomly arise during development to distinguish between twins. "Ultra-deep" means each basepair in the DNA is read multiple times to rule out the possibility that a "mutation" was just a read error.

The introduction of any new technology into the court is always fraught, though. The study published on Eurofins' test, for example, looks at only one pair of twins. Wired also notes it could get especially tricky if your DNA sample comes from blood because identical twins actually share blood in the womb. In Dwayne McNair's case, the sample comes from semen, and prosecutors have said the DNA test identifies Dwayne instead of his twin. A hearing on January 15 will determine if this DNA test is admissible in court.

Whether or not this twin DNA test makes it into the courtroom, you can probably look for it on the next season of Law & Order. [Wired]