Researchers craft fake DNA evidence

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If there's one application of modern genetics that the public has not only accepted but embraced, it's the use of DNA testing in criminal investigations. Courts have accepted DNA evidence as definitive, and it's difficult to imagine a movie or TV show that focuses on law enforcement but declines to use DNA testing as a plot device. The reason is simple: given a valid DNA sample, the tests can match it to its source with probabilities that exclude the rest of our planet's population. Those probabilities still hold, but some researchers have now looked into whether it's possible to fake a valid DNA sample, and they have come up with a disturbing answer: just about any molecular biology lab has the tools to do so.

DNA evidence is appealing largely because it's rigorously scientific: we have empirical data about the prevalence of different genetic variations in the population, and it's easy to calculate the probability of any individual carrying a specific combination of those variants. Look at enough of them, and you can lower that probability to the point where it's less than one in several billion, meaning that DNA's owner is likely to be the only person on earth with that precise combination.

In contrast, as recent reports have indicated, there's not a lot of science to the rest of forensic science. As the authors of the new paper point out, in contrast to the science behind DNA, "other types of forensic evidence, such as ballistics, blood-spatter analysis, and fiber analysis... rely on expert judgment and have limited connection to established science. [DNA] is even considered to be more reliable than eyewitness evidence, which is known to suffer from a relatively high rate of errors."

Which is what makes their report disturbing. The forensic processing pipeline includes a variety of methods to identify likely sources of DNA, ranging from identifying obvious points of contact like the grip of a gun, to locating sources of biological material, such as blood and saliva spatters. DNA is then isolated from these sources. But there's no way to determine if the DNA that actually wound up in or on the sample is actually the same DNA it started with.

As the researchers demonstrate, it's possible to exploit this loophole with a vengeance. Purified DNA can be smeared all over the surface of your choice, such as a gun grip. It's also possible to eliminate the original DNA from blood and saliva samples using a standard piece of lab equipment called a centrifuge, which spins rapidly in order to separate components of liquids based on their density. So, for example, it's possible to spin all the cells that contain DNA out of a saliva sample, or separate the white blood cells out of a blood sample. The liquid that's left behind looks like a valid biological sample, but contains none of the original DNA.

All that's left then is to replace the original DNA. Since most of the forensic tests are performed using a standard set of DNA fragments amplified by PCR, the authors simply took an environmental sample ("blood, dry saliva stains on absorbent paper, skin scrapings, hair, and smoked cigarette butts were collected"), amplified up the same fragments, and then spiked the purified blood and saliva with this DNA. They estimate that a library of about 425 DNA fragments would be enough to fake a match to just about anyone in existing DNA databases—without any DNA from that individual ever being obtained.

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