Faulty Forensics:
Spotlight on
Complex DNA Mixtures
Complex DNA mixtures analyze DNA from three or more people
Complex DNA mixtures can occur in many ways. When numerous people touch an object or surface, their DNA can be transferred.
The average person transfers their DNA dozens of times throughout the day. When complex DNA mixtures are collected from a crime scene, they may contain the DNA of people who shook the hand of a suspect and weren't even there. These complex mixtures are extremely vulnerable to misinterpretation.
Even if there is a small amount of DNA at a crime scene, people who weren't even there may be implicated
It is not uncommon for humans to share DNA.
When there is not enough DNA to get a good sample, scientists can get a result where only part of the DNA matches the suspect. If this result is compared to others in the population, it may match any number of people because there wasn't a full profile.
FACT: Complex DNA mixtures are much less reliable than traditional DNA testing
Traditional DNA testing often relies on the DNA of only one or two people with a larger sample size is left behind. This makes it much easier to separate the samples from each other and identify specific suspects. Complex DNA mixtures with small samples should only be used with caution and other evidence to back up the results.
Humans share a vast majority of our DNA
99%
In January 2020, Fransisco Ortiz was out riding in a car in San Diego, CA with his girlfriend, his girlfriend’s mother, and the mother’s friend, when they were pulled over by police. They were looking for the girlfriend’s brother. Police spotted a handgun in the vehicle and a search ensued. They found 18 grams of methamphetamine along with the gun and Ortiz was charged with illegal possession of both items.
The San Diego Police Department Crime Lab conducted DNA testing on the gun and discovered a complex mixture. The analyst determined that the gun had DNA from five different individuals based on the STRmix software program that the lab used. STRmix is a probabilistic genotyping software designed to analyze complex mixtures from 2+ people. Though STRmix is used nationwide in crime laboratories, questions exist surrounding its developmental and internal reliability.
Ortiz filed a motion to exclude the STRmix DNA analysis from the crime lab, arguing that the mixture had at least six contributors. STRmix developers have done internal studies and determined that the program is reliable only for samples with up to five contributors. When the number of contributors is more than two, the analysis gets more complicated and harder to discern the true number of contributors, if at all. The analyst here determined that Ortiz was a contributor to the DNA mixture found on the gun.Ortiz argued that the analyst in his case reached her conclusion through a judgement call with constraints to the program for such a complex mixture, therefore it should not be allowed as evidence in his case under Federal Rule of Evidence 702.
In June 2024, the United States District Court for the Southern District of California granted Ortiz’s motion and found the STRmix inadmissible under Rule 702 and Rule 403.
Complex DNA Mixtures Case Study:
Fransisco Ortiz
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