NIST Research Contributes to Expanded Forensic DNA Profiles
Written by Rich Press   

This year marks an important milestone in the development of forensic DNA profiling in the United States. Since the FBI’s National DNA Index System, or NDIS, came online in 1998, forensic laboratories in the United States have been generating DNA profiles by analyzing a specific set of 13 genetic markers. On January 1, 2017, the FBI started requiring that all DNA profiles submitted to NDIS be based on 20 markers.

The expansion of the core set of markers was made possible by close collaboration between scientists at the FBI and the National Institute of Standards and Technology (NIST). The additional markers will vastly increase the statistical certainty of DNA identifications and allow investigators to identify suspects that could slip through the cracks today.

This upgrade was necessary in part due to the rapid growth of NDIS, which has expanded to include nearly 16 million profiles related to criminal investigations and 30,000 related to missing persons. As the number of profiles in the system increases, the probability that a search might return a false match goes up. Adding new markers mitigates that risk.

In addition, this upgrade will make international DNA searches more effective by increasing the number of markers that the U.S. system has in common with those of other nations. The number of markers used in both the United States and Europe, for example, will rise from eight to 15.

Partial Profiles

The new markers will also help solve a problem that often comes up in cases that involve degraded DNA. In those cases, forensic analysts can’t always get a read on all 13 markers, and they end up with a partial profile.

“If you’ve got a case where seven markers drop out, the statistics may be too weak to establish an identity,” said Mike Coble, a research geneticist at NIST. When that happens, a perpetrator might escape the notice of investigators and remain free to commit more crimes.

“But if you start with 20 markers, seven can drop out and you might still have enough to establish an identity,” Coble said.

Of the seven new NDIS markers, three were first identified by Coble and his colleagues at NIST. And those three markers were chosen because they are particularly useful in cases that involve degraded DNA.

A genetic marker is a stretch of DNA that occurs at a specific location on a chromosome. Forensic markers don’t code for anything, but they contain a section of genetic code that repeats itself, like a word typed over and over. The number of repeats at each marker varies from person to person, and the chances that two people that are not closely related have the same number of repeats at the original set of 13 markers is less than one in a trillion.

If you lined up the number of repeats for all the markers, you’d have something like a very long social security number that can be used to identify people, and that’s what a DNA profile is. Because we have two copies of each marker—one inherited from each parent—a DNA profile based on 13 markers is 26 numbers long. Since the recent upgrade to 20 markers, DNA profiles in the United States are a series of 40 numbers.

To get those numbers, forensic analysts measure how long the markers are, and then deduce the number of repeats from that length. But DNA is fragile. Heat, sunlight and bacteria can all damage the molecule, and if a break occurs within a marker, scientists can’t measure how long it is. In that case, they’ll get a partial profile.


Until recently, DNA profiles in the U.S. were a series of 26 numbers. If you had only a partial profile, it might not include enough information to establish an identity.


Today, DNA profiles in the U.S. are a series of 40 numbers, making it much more likely that, even with a partial profile, there will be enough information to establish an identity. Pictured here is NIST scientist Mike Coble.


The Search for “Mini-Markers”

But some markers withstand damage better than others. When DNA is damaged, it starts breaking apart, like a ribbon cut to pieces. Shorter markers tend to remain intact during that process just because they’re small pieces already.

But only a few of the markers in the original set of 13 are particularly short. So, in 2004, Coble and his colleagues at NIST set out to find a few new ones.

“We were looking for markers that were short and that showed a lot of variability,” Coble said. “If half the population has the same number of repeats, it won’t be very good for telling people apart.”

They started with a list of more than 1,000 candidate markers, most of which were unknown when the original set of 13 markers was chosen a decade earlier. After running thousands of tests, including tests using artificially degraded DNA, they found 27 “mini-markers” that worked.

Of those 27, three are among the seven new NDIS markers. The other four new markers aren’t minis, but they have exceptionally high variability.

Much of the research that yielded the mini-markers grew out of the effort to identify the victims of the 9/11 terror attacks. That effort was complicated by the fact that fires smoldered for months in the rubble at Ground Zero and, because heat damages DNA, the remains recovered there often yielded partial profiles. To help resolve those profiles, NIST scientist John Butler worked with colleagues at the New York City Medical Examiner’s Office to develop new methods for working with badly degraded DNA—methods that made it possible to identify remains that otherwise would have never been returned to the victims’ families.

That research was the precursor to the search for mini-markers at NIST. More recently, NIST worked closely with the FBI to validate the new 20-marker profiling kits that forensic labs use to generate DNA profiles. The agencies coordinated a series of trials in which crime labs used the kits against test DNA with known profiles to ensure that the results were accurate and that kits from different manufacturers produced equivalent results.

For crime victims seeking justice, for defendants seeking a fair trial, and for families looking for missing loved ones, a lot depends on the accuracy and reliability of DNA profiles. The expansion to 20 markers will ensure that, even as the national DNA database continues to grow, this technology will remain an incredibly powerful tool for solving cases.


About the Author

Rich Press, is a science writer at NIST.

 
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