In late April 2018, police in California arrested an alleged rapist and killer, Joseph James DeAngelo, for crimes they claim he committed 30 to 40 years ago.
He is a suspect in 13 murders and 51 rapes committed across California between 1976 and 1986.
DeAngelo is a former police officer who has never made his DNA available to anyone…but his DNA caused his arrest. How can that be? Apparently, detectives linked him to DNA left at crime scenes decades ago, through a genealogy database designed to trace family histories.
DNA sent to such a site by a relative of the murderer may have provided the crucial connective clue to police.
The online news outlet, Ars Technica, explained how DNA tracing works:
Your family controls your privacy
It’s probably safe to assume that Joseph James DeAngelo has never placed any DNA test results online. So how were the police able to identify him?
Because you and your relatives share ancestors, you also share DNA. The fraction will depend on the degree of relatedness; siblings will typically share half of their genomes, while first cousins only share 1/8th.
But the pattern of shared DNA is even more telling. We inherit chromosomes, which are large, single molecules of DNA. Over time, these molecules will gradually exchange segments with chromosomes inherited from other individuals. But the process is slow, so long stretches of your ancestors’ original chromosomes are preserved intact for many generations.
As a result, the 1/8 of a genome you share with a cousin isn’t mixed randomly throughout your chromosomes. Instead, it’s found in large chunks that are identical, interspersed with equally large chunks that are unrelated.
By identifying these chunks using DNA data, you can estimate the degree of relatedness between any two individuals (if there is any). As people make more of their DNA available through public-facing services, the prospect of identifying other family members online goes up.
This has led to many happy stories of long-lost family members found and the discovery of family members people never knew they had.
But it also has the potential to allow people who don’t want to be found to be identified – and their reasons for not wanting to be found may be nowhere near as nefarious as DeAngelo’s.
DNA testing has reached a level of popularity that ensures that many of us have parts of our genome available online, even if we’ve never spit into a test tube ourselves. And the decisions on who gets access to that data may be in the hands of family members we’ve never spoken to – or didn’t even know existed.
And, once a few family members are identified through DNA, public records can be used to identify other family members, as was done in DeAngelo’s case.