How forensic genetic genealogy works: the type of DNA test used, consumer genealogy databases and consent, building family trees to find a suspect, confirming with standard DNA, famous cases, identifying unknown remains, and the privacy debate.
In April 2018, US police arrested Joseph James DeAngelo, the "Golden State Killer", for crimes committed decades earlier. The breakthrough didn't come from a police database: investigators found distant relatives through a consumer genealogy website. Since then, forensic genetic genealogy (FGG) has helped solve hundreds of cold cases. Here's how it works, and why it's controversial.
Two kinds of DNA testing
- Traditional forensic DNA profiling looks at a small number of locations in the genome called STRs. Police databases, such as the US CODIS system or the UK's National DNA Database, store these profiles. They can identify a person already in the database, or sometimes a close relative, but little more.
- Genealogy testing examines hundreds of thousands of SNPs (single-letter variations across the genome). Comparing SNPs reveals how much DNA two people share, and so how closely related they're likely to be, including distant cousins.
How a case is solved?
- Get a SNP profile from crime scene DNA. This often needs specialist labs, especially for old or degraded samples.
- Upload to a genealogy database that permits law enforcement use. Most major consumer companies don't allow it; a few services do, for users who have opted in.
- Find matches: usually second, third or more distant cousins.
- Build family trees from public records such as births, marriages, obituaries and social media, working back to shared ancestors and then forward to find people who could be the source.
- Narrow it down using age, location and other evidence.
- Confirm with standard DNA: police typically obtain a sample from the suspect, often from discarded items, and compare it with the crime scene profile. A genealogy lead alone isn't proof.
Identifying the unknown dead
FGG isn't only used to find offenders. Volunteer organisations such as the DNA Doe Project use it to identify unknown human remains, giving names back to people and answers to families who have sometimes waited decades.
The privacy debate
When you upload your DNA, you're also sharing information about relatives who never agreed to it. Critics raise several concerns:
- Consent: after controversy in 2019, the database GEDmatch changed its policy so users must opt in to law enforcement matching.
- Scope creep: debate continues over whether FGG should be limited to violent crimes and identifying remains.
- Errors: family tree research can point to the wrong branch, which is why confirmation testing matters.
- Regulation: some US states, such as Maryland and Montana, have passed laws controlling its use, and the US Department of Justice has an interim policy.
And in the UK?
FGG has been used far more widely in the US. Its use by UK police has been limited and raises questions under data protection and human rights law, along with the different consent rules of genealogy companies. If you've taken a DNA test, check your provider's privacy settings to see whether, and how, police could access your data.
Should you opt in?
It's a personal choice. Opting in can help solve serious crimes and identify the dead; opting out protects your and your family's genetic privacy. Whatever you choose, read the terms, and remember your decision affects relatives too.
You can usually change your setting later, and most services let you delete your data entirely. Deleting it removes it from future searches, but it can't undo matches that have already been made.
Sources and further reading
- DNA Doe Project
- GEDmatch
- US Department of Justice: Interim policy on forensic genetic genealogy
- NIST: Forensic science
Facts checked on 20 October 2026. Spotted an error? Tell us and we will correct it.
Frequently asked questions
What should you know about Identifying the unknown dead?
FGG isn't only used to find offenders. Volunteer organisations such as the DNA Doe Project use it to identify unknown human remains, giving names back to people and answers to families who have sometimes waited decades.
What should you know about the privacy debate?
When you upload your DNA, you're also sharing information about relatives who never agreed to it.
And in the UK?
FGG has been used far more widely in the US. Its use by UK police has been limited and raises questions under data protection and human rights law, along with the different consent rules of genealogy companies.
Should you opt in?
It's a personal choice. Opting in can help solve serious crimes and identify the dead; opting out protects your and your family's genetic privacy.