Not all DNA evidence is equal

For three decades, forensic DNA evidence has been a valuable tool in criminal investigations, incriminating or exonerating suspects. Matching a defendant’s genetic material with a sample found on a weapon or at a crime scene has proved extremely persuasive with judges and juries.

But not all DNA evidence is equal, reports the New York Times. 

Sometimes it’s clear: blood or semen identifies a single person. If it’s just a few skin cells left on an object, or if it contains more than one person’s genetic material, it can be more ambiguous. In such situations, labs used to report that the results were inconclusive, or the defendant could not be excluded from the mix.

New types of DNA analysis have been introduced in recent years to interpret trace amounts or complex mixtures, spawning an industry of testing tools, chemical kits and software. As analysis has become more complex, the techniques and results are coming under fire nationwide.

In the past three years, flaws in DNA methods have temporarily shut down testing in public crime labs in Austin, Tex., and Washington. Lab analysts “make it seem like it’s a completely objective process,” said Bicka Barlow, a lawyer in California with a master’s degree in genetics and molecular biology. “But I’m 100 percent convinced that there are many people who are incarcerated who were convicted with DNA evidence who are innocent.”

The two techniques that New York’s lab introduced were the “high-sensitivity testing” of trace DNA amounts, and the Forensic Statistical Tool, or FST, in which software calculates the likelihood that a suspect’s genetic material is present in a complicated mixture of several people’s DNA. By its own estimate, the lab has used high-sensitivity DNA testing to analyze evidence samples in 3,450 cases over the past 11 years, and the FST in 1,350 cases over the past six. Cases in which both methods were used may be counted in both totals.

A New York crime lab DNA analysis methods are under the microscope, with scientists questioning their validity. In court testimony, a former lab official said she was fired for criticizing one method, and a former member of the New York State Commission on Forensic Science said he had been wrong when he approved their use. The first expert witness allowed by a judge to examine the software source code behind one technique recently concluded that its accuracy “should be seriously questioned.”

Earlier this year, the lab shelved the two methods and replaced them with newer, more broadly used technology.

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