With the use of DNA evidence increasing across the United States, DNA labs are using probabilistic genotyping to analyze hard-to-interpret samples. However, some scientists and lawyers worry that the privately held computer code behind these tools is limiting its reliability and hindering due process.
Traditional DNA analysis is challenged when there are multiple
contributors to a sample or the quantity of DNA recovered is too small.
Without a better analytical tool, these samples are often inconclusive,
says Dan E. Krane, a professor of biological science at Wright State
Probabilistic genotyping is not a technique that defines the sample
itself; rather it is an interpretive software that runs multiple
scenarios—like the risk analysis tools used in finance—to examine the
sample. This contrasts with traditional DNA analysis, which assesses
whether a DNA type is present or absent.
Bjorn Sutherland, forensic development manager at the New
Zealand-based Institute of Environmental Science and Research—a
probabilistic genotyping company—says that his software, STRMix, “enables users to compare the
results against a person or persons of interest and calculate a
statistic, or ‘likelihood ratio,’ of the strength of the match.”
By leveraging computer processing, probabilistic genotyping “gives us
more information to work with,” says Chris Lindberg, a deputy district
attorney in San Diego. Many, like Lindberg, are excited for this
technology because it analyzes samples in a way that would have been too
labor intensive previously. The cost of these tools varies by company
and number of licenses purchased.
Last year, San Diego joined jurisdictions in Indiana, Louisiana and
New York, among others, deploying this technology in its investigations.
Sutherland says this technology has been around for less than 10 years,
but the statistical models the tools use have been around for decades.
This science has created a cottage industry. Besides STRmix, those
receiving the most attention in the U.S. are the Forensic Statistical
Tool, used by the Office of the Medical Examiner in New York, and
TrueAllele, created by the Pittsburgh-based company Cybergenetics.
The September 2016 PCAST report on forensics noted that “probabilistic genotyping
software programs clearly represent a major improvement over purely
subjective interpretation.” However, the report added, “careful
scrutiny” is still needed to determine whether methods are
scientifically valid and if the software correctly implements those
methods. The report clarifies that analyzing the software “is
particularly important because the programs employ different
mathematical algorithms and can yield different results for the same mixture profile.”