Probabilistic genotyping and the power of modern DNA technology: A case study in overturning a wrongful conviction for murder in Texas

  • Angie Ambers, PhD

    Assistant Director, Henry C. Lee Institute of Forensic Science, Associate Professor, Forensic Geneticist, Henry C. Lee College of Criminal Justice & Forensic Sciences, University of New Haven


In late 2019, nearly a decade into a life sentence, Lydell Grant was released from a Texas prison after being convicted of a murder that he did not commit. The victim, Aaron Scheerhorn, was stabbed multiple times in a parking lot outside of a bar in Houston. A DNA mixture was recovered from the victim’s fingernails but was too complex to analyze at the time of the original trial. Mr. Grant’s conviction resulted primarily based on the testimony of six eyewitnesses, all of whom identified him as the killer. During the trial and the entirety of his imprisonment, Grant vehemently denied his involvement in the crime. In 2018, forensic DNA expert Dr. Angie Ambers and a legal team from the Innocence Project of Texas requested access to evidentiary samples collected from the victim during autopsy. Mixed DNA data generated from the victim’s fingernails was sent for analysis using a “probabilistic genotyping” software program called TrueAllele®. Analysis of the raw data from this DNA mixture not only excluded Mr. Grant as the source of the foreign DNA, but resulted in a CODIS database hit that identified the actual murderer of Mr. Scheerhorn. This presentation will provide an overview of Grant’s case, as well as a discussion of subjectivity issues with previous DNA mixture interpretation approaches and the improved (more  objective) analytical capabilities provided by probabilistic genotyping software programs.

Learning Objectives:

1. Identify issues regarding DNA mixture interpretation

2. Learn about current probabilistic genotyping programs for forensic casework

3. Understand how probabilistic genotyping can assist with post-conviction DNA testing

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