Forensic DNA, PCR, and a Contested Sample

Edward Blake has been a private forensic consultant who ran Forensic Science Associates in California. He was retained to examine evidence in high-profile trials, and he conducted DNA testing in the Roger Keith Coleman case. He kept a frozen vaginal swab sample in his laboratory and resisted returning it to Virginia for retesting. Journalists and advocates have sought new testing while the sample remained in California, and legal disputes over custody of the evidence were litigated.

The polymerase chain reaction (PCR) has been adopted widely because it has allowed analysts to amplify very small or degraded DNA samples, shortening laboratory time and increasing sensitivity. PCR-based methods have been used in many post-conviction reviews, and they have helped to identify or exclude suspects when old evidence could be re-examined. The National Registry of Exonerations and advocacy groups report that over 600 people in the United States have been exonerated after DNA testing. Many courts and laboratories now consider long-term preservation of biological evidence to be essential for justice.