Case report: Coincidental inclusion in a 17-locus Y-STR mixture, wrongful conviction and exoneration

- A coincidental match to a 17-locus Y-STR mixture led to a wrongful conviction.
- Expansion from 17 to 23 loci Y-STR test, excluded suspect and led to an exoneration.
- Likelihood ratios must consider all relevant hypotheses.

We report the case of a suspect (Suspect-3) who was convicted (and later exonerated) of participating in the multiple-attacker rape of two women. The forensic evidence against him was his inclusion in a 17-marker Y-STR mixture isolated from semen on one victim's clothing. The DNA inclusion produced a match statistic with a combined probability of inclusion of 1 in 741, and a Likelihood Ratio of 3296. While the defense team was told that Suspect-3 was included in the semen DNA mixture, they were not told that all of the Y-STR alleles could also be explained by just the other two accused attackers' haplotypes. Suspect-3 was subsequently freed after the Taiwan Association for Innocence requested re-examination of the incriminating mixed DNA sample. The Criminal Investigation Bureau was then able to exclude him using an extended set of Y-STR markers (23 loci), leading to his exoneration.