Crime scene DNA is typically recovered from blood or semen stains, but the DNA that exonerated members of JonBenet Ramsey's family came from invisible skin cells.
This so-called "touch DNA" is left behind when people touch things, because they naturally shed skin cells that contain the genetic material. In this case, the new DNA was recovered by guessing where JonBenet's killer might have handled the long johns she was wearing.
"It's not a stain, you can't see it," said Angela Williamson, director of forensic casework at the company that recovered the DNA material, Bode Technology Group in Lorton, Virginia.
To find such DNA, "you have to have a good idea of where someone has been touched, or in this case, where you think the suspect would have touched" JonBenet's clothing, she said.
Investigators suggested that somebody pulling down her pants would have touched the waistband and the sides of the long johns, Williamson said. So Bode scientists scraped those areas with a sharp blade to see if they could find DNA.
While the amount of DNA they found was much less than would appear in a stain, there was enough that it was processed in the routine way for analysis, Williamson said. (In other cases, so-called "low copy number DNA" has to be processed in a different way).
DNA from two sites on the long johns matched genetic material from an unknown male that had previously been recovered from blood in JonBenet's underpants. The matching DNA from three places on two articles of JonBenet's clothing convinced the district attorney that it belonged to the killer, and hadn't been left accidentally by a third party.
Source:Xinhua/Agencies
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