New software will help York County crime lab untangle complex DNA mixtures


A printout marked with a series of numbers and colorful zig-zags may look like a foreign language to some people.

But to Cristy Kissel, who leads the York County Sheriff’s Office DNA laboratory, those lines and numbers represent the extent to which people’s DNA is present in evidence collected during criminal investigations.

Thanks to a new software recently purchased by the Sheriff’s Office, Kissel and her co-workers will be better able to distinguish a specific DNA profile in samples that include multiple profiles.

The lab routinely receives pieces of evidence gathered by investigators from crime scenes.

“If we have a suspect or elimination samples, we’ll try to compare those to those (evidence) samples to either eliminate them or include those individuals,” she said. “If we don’t have enough profiles from any known individuals, then we try to take that DNA profile and search it through CODIS.”

CODIS is the Combined DNA Index System, which serves as the FBI’s criminal justice DNA database.

Kissel said a majority of the samples the lab receives have “mixture profiles,” meaning there are multiple DNA profiles on the item – for example, a door knob or steering wheel. Even blood samples may not be cut-and-dry becasue the sample might be very small and lifted from an object that multiple people have touched.

“We tend to get mixture profiles, where we have anywhere from two to three or four individuals all contributing to that profile,” she said. “There’s no real instant way to know which profile is from which person other than looking at the profile and trying to either mathematically, or through the amount of DNA there, determine if we can pull out a major profile or profile donor.”

Doing that usually takes hours just to ascertain one profile in one piece of evidence, Kissel said. But the new probabilistic genotyping software does that work, with greater accuracy, in a matter of minutes.

Because it would take weeks to type out an entire DNA strand, lab technicians look at 15 specific locations of a strand, each of which is called a locus, Kissel said.

“It goes through each locus and takes the probabilities that this person could be in this mixture with this profile at this locus,” she said. “And once it does that for each locus, it can go back and take all of them and say, ‘What is the probability this person and this amount of a donor could make this profile happen?’”

When the software finds the best match fit, the program generates the series of line graphs and numbers indicating how confident it is, Kissel said. The software also tells technicians if it can’t distinguish a profile.

“We’ll know this profile is just not usable,” Kissel said. “We don’t have to spend hours poring over it, and the computer can tell us in five or 10 minutes that this profile is just not usable and we need to move on.”

Backlogs are common at many labs, including York County’s, Kissel said. The software, which cost nearly $50,000, was purchased with a federal grant aimed at reducing laboratory backlogs.

A similar software called TrueAllele has put the Richland County Sheriff’s Department DNA lab “on the cutting edge of forensic genotyping” by helping technicians better examine complex DNA mixtures, according to Dr. Gray Amick, director of Richland County’s lab.

“Many DNA profiles generated these days from touch samples result in mixtures and TrueAllele can objectively determine contributors’ genotypes, mixture weights and the likelihood of their presence,” Amick said. “As more and more samples are run and more and more mixtures are developed from them, TrueAllele has and will become an indispensable tool for forensic DNA laboratories.”

York County is one of only a handful of agencies in the state that has this type of software, Kissel said. They are still in the training and implementation stages, including training employees on how to testify about the software once cases make it to court. It will be at least a couple of more months before the software is being used in cases.

“It’s starting to trickle into the labs, but not all have used it yet,” Kissel said. “The FBI hasn’t required it for our participation in CODIS yet.”

Teddy Kulmala: 803-329-4082, @teddy_kulmala