DNA and Genetic Genealogy: Solving the Unsolvable
Genetic genealogy is solving decades-old cold cases at an unprecedented rate. Here's how the science works.
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The Golden State Killer Breakthrough
In 2018, investigators arrested Joseph James DeAngelo for the Golden State Killer crimes—rapes and murders spanning the 1970s-80s. How did they catch him after 40 years?
The Process:
1. Upload crime scene DNA to GEDmatch (public genealogy database)
2. Find distant relatives who share DNA
3. Build family trees for matches
4. Identify common ancestors
5. Work down to living descendants
6. Narrow to suspect fitting crime profile
7. Confirm with direct DNA sample
This technique has since solved 200+ cold cases.
How Genetic Genealogy Works
- We share DNA with relatives
- 50% with parents/children
- 25% with grandparents, aunts, uncles
- 12.5% with first cousins
- Distant relatives: Small but detectable amounts
The Process
Investigators don't need the killer's DNA in a database—just a relative's. From a third cousin match, genealogists can trace family trees back to find who could have left the DNA.
Cases Solved by Genetic Genealogy
Golden State Killer (2018)
40+ years unsolved. Joseph DeAngelo caught through GEDmatch.
Bear Brook Murders (2019)
Unidentified victims and killer both identified through genealogy.
April Tinsley (2018)
8-year-old murdered in 1988. Killer caught 30 years later.
Hundreds More
Genetic genealogy is now standard practice for cold case units.
Privacy Debates
- Relatives didn't consent to their DNA catching criminals
- What else might this data be used for?
- Should there be limits on familial searching?
Courts have generally allowed the technique, but debates continue.