How does a DNA registry help identify a criminal?
July 27, 2020 9:04 AM Subscribe
Reading about serial killer Joseph DeAngelo, I am curious how an ancestry site match is what pinpointed him as the killer.
It sounds like they took a DNA sample from a 1970s crime scene, used it for a fake profile on GedMatch (how?), then found a fourth cousin who happened to have done a DNA test.
This led them to DeAngelo as a relative... but how? Clearly, knowing a family tree would help narrow down a huge population to a much smaller group of distantly related people in the right age range and city... but how do you track down a random fourth cousin?
Then they surveilled DeAngelo and swabbed something in his garbage to get a current DNA sample that matched the crime scene sample.
My question is that the specific ancestry sites claim law enforcement did not contact them... but if the DNA collection technique for most of these sites is a recent cheek swab, how could law enforcement covertly submit a 40 year old crime scene sample which I'm assuming was not cheek cells? It seems to me that the site and the investigators must have collaborated?
It sounds like they took a DNA sample from a 1970s crime scene, used it for a fake profile on GedMatch (how?), then found a fourth cousin who happened to have done a DNA test.
This led them to DeAngelo as a relative... but how? Clearly, knowing a family tree would help narrow down a huge population to a much smaller group of distantly related people in the right age range and city... but how do you track down a random fourth cousin?
Then they surveilled DeAngelo and swabbed something in his garbage to get a current DNA sample that matched the crime scene sample.
My question is that the specific ancestry sites claim law enforcement did not contact them... but if the DNA collection technique for most of these sites is a recent cheek swab, how could law enforcement covertly submit a 40 year old crime scene sample which I'm assuming was not cheek cells? It seems to me that the site and the investigators must have collaborated?
From the genealogy research side of it, I've done this with a friend's DNA and found their birth family using a distant cousin that was a DNA match. Basically, you build the family tree backwards. If you know someone is your fourth cousin you determine how far back you most likely share a common ancestor (however many greats grandparent). Then you build the family tree for the known fourth cousin. You determine all their great greats back to the common ancestor generation And then you trace all those separate grandparent lines to the present day.
By tracing each individual line I was able to determine which family was most likely the birth family based on their location and one other bit of info I had.
posted by ilovewinter at 9:52 AM on July 27, 2020 [4 favorites]
By tracing each individual line I was able to determine which family was most likely the birth family based on their location and one other bit of info I had.
posted by ilovewinter at 9:52 AM on July 27, 2020 [4 favorites]
The technique was covered in HBO's 5th segment of "I'll be gone in the dark". It included converting the DNA profile into a digital format for the match.
The tree matching is also included in the explanation. Ultimately, the DNA expert read that DeAngelo had blue eyes from a newspaper article she read and he was the only person from the six remaining relatives that had blue eyes. So, in part, really stunning detective work.
posted by effluvia at 9:56 AM on July 27, 2020 [6 favorites]
The tree matching is also included in the explanation. Ultimately, the DNA expert read that DeAngelo had blue eyes from a newspaper article she read and he was the only person from the six remaining relatives that had blue eyes. So, in part, really stunning detective work.
posted by effluvia at 9:56 AM on July 27, 2020 [6 favorites]
if the DNA collection technique for most of these sites is a recent cheek swab
It's actually usually a saliva sample (you spit into a tube -- a lot of spit). Not sure if that affects the answers, but it might.
posted by fiercecupcake at 10:02 AM on July 27, 2020
It's actually usually a saliva sample (you spit into a tube -- a lot of spit). Not sure if that affects the answers, but it might.
posted by fiercecupcake at 10:02 AM on July 27, 2020
how could law enforcement covertly submit a 40 year old crime scene sample which I'm assuming was not cheek cells? It seems to me that the site and the investigators must have collaborated?
My sister works in a crime lab. Unsolved crimes, especially high profile ones, often have a lot of stored DNA waiting in freezers, waiting for a day when DNA science and matching stuff is up to the task of doing some of the work that was often more hand-done. So I don't know this crime but in a case where there was any DNA from the person who committed the crime (skin cells, hair samples with roots attached, fluids etc), they can process it into GEDMatch format, upload it as if they were that person ":Hey I just swabbed myself!" and then look for relatives.
If this is the kind of thing you're interested in, the Bear Brook Murder podcast talks a LOT about how they figured out the identity of unknown victims and pointed towards a killer through some DNA databases plus good guessing (as effluvia says)
posted by jessamyn at 10:22 AM on July 27, 2020 [1 favorite]
My sister works in a crime lab. Unsolved crimes, especially high profile ones, often have a lot of stored DNA waiting in freezers, waiting for a day when DNA science and matching stuff is up to the task of doing some of the work that was often more hand-done. So I don't know this crime but in a case where there was any DNA from the person who committed the crime (skin cells, hair samples with roots attached, fluids etc), they can process it into GEDMatch format, upload it as if they were that person ":Hey I just swabbed myself!" and then look for relatives.
If this is the kind of thing you're interested in, the Bear Brook Murder podcast talks a LOT about how they figured out the identity of unknown victims and pointed towards a killer through some DNA databases plus good guessing (as effluvia says)
posted by jessamyn at 10:22 AM on July 27, 2020 [1 favorite]
Yea, they literally covered this in the recent HBO episode. Though, my memory was that the actual DNA profile said the suspect likely had blue or hazel eyes and early balding and only one remaining suspect had blue eyes.
From my memory of watching the episode last night: They submitted to GEDmatch because they could upload a digital DNA profile, without needing a tube of saliva like other sites. One of the labs had plenty of remaining DNA to be profiled and submitted. Then they had a few matches that were likely 4th cousins. They did genealogy research to make a tree, and match to the suspect possibilities. (Men, the right age at the right time.) Then they narrowed down from there. Then tested the suspect's trash after they had a hit and got a match. Etc.
(Please correct if anything is wrong.)
posted by Crystalinne at 11:56 AM on July 27, 2020 [1 favorite]
From my memory of watching the episode last night: They submitted to GEDmatch because they could upload a digital DNA profile, without needing a tube of saliva like other sites. One of the labs had plenty of remaining DNA to be profiled and submitted. Then they had a few matches that were likely 4th cousins. They did genealogy research to make a tree, and match to the suspect possibilities. (Men, the right age at the right time.) Then they narrowed down from there. Then tested the suspect's trash after they had a hit and got a match. Etc.
(Please correct if anything is wrong.)
posted by Crystalinne at 11:56 AM on July 27, 2020 [1 favorite]
The book “the Lost Family” by Libby Copeland also goes into this and other ethics/questions around DNA ancestry. It was a good relatively fast read and really emphasized that submitting your DNA exposes your whole genetic line, making issues of consent complicated...
posted by five_cents at 3:54 PM on July 27, 2020
posted by five_cents at 3:54 PM on July 27, 2020
A couple of years ago, I did this for myself when I found out I was conceived using an anonymous sperm donor. As ilovewinter said, it involved building out family trees for the closest-matching cousins.
In my case, on my father's side I had several third-cousin matches. I found two cousins who were related to each other, and built their trees out to find the place where their two family trees intersected (their most recent common ancestor). I then found two more cousins who were related to each other, but not to the first pair, and built out their trees to figure out where they intersected. The intersections of the two cousin pairs' trees were my great-great (-great?) grandparents, one on each side of my father's family.
Then, I built out trees for those great-great grandparents, and found where those two trees intersected -- where they were related to each other. That intersection had to be my grandmother and grandfather, and one of their children had to be my father. Helpfully, my grandparents only had one son.
posted by woodvine at 4:27 AM on July 29, 2020 [2 favorites]
In my case, on my father's side I had several third-cousin matches. I found two cousins who were related to each other, and built their trees out to find the place where their two family trees intersected (their most recent common ancestor). I then found two more cousins who were related to each other, but not to the first pair, and built out their trees to figure out where they intersected. The intersections of the two cousin pairs' trees were my great-great (-great?) grandparents, one on each side of my father's family.
Then, I built out trees for those great-great grandparents, and found where those two trees intersected -- where they were related to each other. That intersection had to be my grandmother and grandfather, and one of their children had to be my father. Helpfully, my grandparents only had one son.
posted by woodvine at 4:27 AM on July 29, 2020 [2 favorites]
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i don't know the details of GEDMatch but I believe its one of the sites that allow you to just upload a DNA file that you exported from some other site. So the investigators could have done the DNA analysis by themselves and then got it into the right format and imported it into GEDMatch.
posted by vacapinta at 9:17 AM on July 27, 2020 [3 favorites]