You send away for one of those mail-in DNA tests, spit in the tube, mail it back to the company and get your results, hoping to connect with members of your family tree or find out if you’re really a quarter Italian like your grandma claims.
But instead of matching with a second cousin or great aunt, you match with a police officer.
That’s what happened in April 2018 when police used a genealogy database to identify a suspect in the long-cold Golden State Killer case. Police used DNA collected from a crime scene to find people with similar DNA, create a family tree and track down potential suspects.
It worked: Police identified and arrested Joseph James DeAngelo Jr., a former police officer, after he had been living under the radar for decades. He now faces more than a dozen murder and rape-related charges for crimes committed in the 1970s and 80s.
As one of the first major cold cases involving a genealogy database, the Golden State Killer case marks a turning point in the power of DNA to solve crimes. But the consequences of that power may not all be positive.
How DNA is used to solve crimes
Amongst all humans, 99.9 percent of our DNA is identical. The differences in our DNA, called polymorphisms, are what make individuals unique and what make DNA testing possible.
Finding a single polymorphism in DNA left at a crime scene and using it to try to match the DNA with a suspect isn’t effective, however, since family members and even strangers can share DNA markers. To be accurate, scientists examine at least 20 different markers before making any conclusions.
DNA’s stability makes it (when used correctly) highly effective for definitive crime-solving, says Celeste Berg, Ph.D., a professor of genome sciences at University of Washington School of Medicine. DNA that is sufficiently preserved breaks down very slowly, allowing plenty of time for analysis.
DNA can be used for more than just proving someone’s guilt. Just this year, scientists were able to analyze 100-year-old DNA and identify the remains of the fallen Romanov family, who were killed in 1918.