THE PROBLEM WITH “TOUCH DNA”. Both Shaer and Andrey Smith tell the story of the “perfect alibi” that I posted on here. (a man who was in police custody at the time of a murder was accused of the crime because the paramedics who brought him to the hospital brought his DNA to the crime scene two hours later). Shaer also tells the story of the Phantom of Heilbronn, whose DNA had been found at a number of crime scenes. The solution to the mystery was that the DNA came from a worker at the factory which made the swabs that were used to collect DNA. I posted on the mystery of the “Woman without a Face” here.

The colorful examples in the last paragraph illustrate a general problem with “touch DNA” (the name for traces of DNA from the skin). Touch DNA can spread easily so that touch DNA found at a crime scene may have taken unusual routes to get there.. Audley Smith cites research in the Journal of Forensic Sciences that showed that “a person who uses a steak knife after shaking hands with another person transfers that person’s DNA onto the handle. In fact, in a fifth of the samples [the researcher] collected, the person identified as the main contributor of DNA never touched the knife.”

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