DNA Testing Under Scrutiny
While Josiah Sutton was sitting in prison on rape charges, his mother was thanking God.
He had been convicted years earlier on DNA evidence, but she knew he was innocent. When she learned about problems at the crime lab, she also knew that her prayers had been answered.
"Thank you, God!" she recalled in a report about false DNA testing. His case and many like it are changing the way labs analyze DNA.
DNA Mixtures
In Sutton's case, the lab had trouble testing a DNA mixture. There were three samples -- one from the victim and two from the defendants. The results were enough for a conviction, and then later a reversal.
According to the National Institute of Standards and Technology Testing, it's a common problem. DNA mixtures are complicated, and many labs are not equipped to handle them. The NIST is undertaking a study to address the problem.
DNA analysis has proven to be very reliable forensic evidence if blood, semen or other biological evidence is left at a crime by one or two people. But with DNA mixtures, not so much.
"Touch" DNA
The biggest problem occurs with "touch" DNA. Investigators routinely swipe doorknobs and other surfaces for property crimes, but many people may have touched such surfaces.
"Some labs won't do anything with that kind of evidence," said DNA expert John Butler. "Other labs will go too far in trying to interpret it."
Butler told the ABA Journal that single-source DNA testing is like doing basic arithmetic; mixture DNA is like doing calculus. Labs are not prepared for the more complex testing, he said.
"So you're going into a final exam on calculus, but you've only done homework on algebra and basic arithmetic," he posed. "Are you going to pass that exam?"
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