The DNA Odds Change When Evidence Comes From More Than One Person


When a single DNA sample is compared to a suspect, DNA is the gold standard of forensic evidence. When DNA evidence includes mixtures of DNA from multiple sources, the scientific community is still figuring things out, says the Grits for Breakfast blog. It takes years for new knowledge to pass down from the highest-end basic research to the work tables of practicing DNA analysts at front-line crime labs. Improvements in DNA science and interpretation techniques have caused practitioners to change how they calculate probabilities when it comes to singling out a defendant based on DNA mixtures when biological evidence includes samples from more than one person.

In the most extreme instance, the new method reduced a one in a billion probability that evidence matched a particular suspect to around one in 50, the Texas Forensic Science Commission’s Lynn Garcia told the Texas Criminal Justice Integrity Unit last week. Garcia said the world’s leading experts met in Poland last week to discuss the issues. This all began when the FBI identified a handful of errors in its CODIS DNA database (51 problems out of tens of thousand of entries) and issued a public notice saying that the change in any probabilities affected would be insignificant.

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