Left to right: Shih-Hsiang (Taiwan Association for Innocence), Chen Long-Qi (exoneree), Greg Hampikian (Boise State University, Idaho Innocence Project) and Joy Chen (Taiwan Association for Innocence). (Photo: Courtesy of Greg Hampikian)

Two women were raped in the early morning of May 25, 2009 in Taiwan. Based on a DNA mixture in the underwear of one of the victims, three men who had been with them in those hours were convicted of raping them.

But in a chance occurrence, the Y-STR mixture in the underwear was provided solely by two of the suspects—and the third suspect was only exonerated when a fuller profile of the DNA showed he was excluded from the mixture, as reported in the latest issue of the journal Forensic Science International: Genetics.

The DNA profile of the third suspect (Suspect 3), or men who were in the same paternal line, “cannot be excluded” from the mixed DNA sample collected from the semen in the underpants, according to one of the prosecution’s forensic reports during the trial. Suspect 3 was convicted and locked up in 2012.

But the Taiwan Association for Innocence took up the case the next year. Upon the advice of international experts, they sought the electropherogram of the Y-STR DNA test. Based on re-analysis of those results, they filed a motion for re-trial in 2013.

They found that the 17 loci from the Y-STR results presented at trial were just part of the story.

“It could have come from all three defendants (including Suspect 3), or from just two defendants (not Suspect 3), because all of Suspect 3’s alleles are found in the combination of the other two defendants,” they write. “In fact, Suspect 3 is the only one of the three suspects whose alleles can be completely removed from the interpretation, leaving all the mixture’s alleles explained by the remaining two suspects.”

A further look at 23 loci available after the trial found that Suspect 3 was actually excluded through the additional data, according to the report.

The chart for the Y-STR results shows how the third suspect was ruled to be part of the DNA mixture with the two other men who were convicted of raping two women. The 17 loci showed how many genes he shared with the two other defendants. But a 23-loci analysis done afterward later ruled him out as being part of the semen mixture in one of the victim’s underwear. (Chart: Greg Hampikian, Boise State University)

The Y-STR mixture would match 1 in 741 of all men in the Han Chinese DNA database. The likelihood ratio placed it 3,296 times more likely that Suspect 3 contributed to the mixture, as opposed to not contributing to that mixture.

But that less-likely scenario was indeed the case, based on the additional genetic information, the DNA experts concluded.

The Taichung High Court granted retrial a week after the results were produced—and then reversed the guilty verdict in March 2014.

The authors of the journal include Greg Hampikian, of Boise State University, who is also the director of the Idaho Innocence Project, as well as Shih-Shian Lo of the Taiwan Association for Innocence and Kuo-Lan Liu of the Taiwanese Criminal Investigation Bureau.

Hampikian discussed the case during a TEDx talk he gave in 2012 titled “Forensic DNA Mixups.”

The authors conclude that Y-STR mixtures, owing to the unique correlation to paternity at population levels, should only be used to exclude suspects—and not include them.

“The possibility that an assailant leaves no trace is a question that cannot be settled by science, as one cannot generally prove a negative,” they write.

“A well-known aphorism in forensic science states, ‘the absence of evidence, is not the evidence of absence,’” they conclude, “but for the innocent person absence (exclusion) is the only evidence their DNA can provide.”

Y-STR genetic profiles, which are passed among males, have been a source of great investigative breakthroughs. For instance, the “Grim Sleeper” serial killer Lonnie Franklin Jr. was caught when his son’s DNA in a database pointed back to his role in the killings. The killer known as China’s “Jack the Ripper” was caught in August 2016 after killing at least 11 women and girls, and spending decades at large. Y-STR analysis brought authorities to 52-year-old Gao Chengyong’s doorstep, after his uncle was placed under house arrest and his DNA was entered into a database.