molly1255
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A lot of adoptees use these sites to try and find birth family members.
If you send your DNA in, and you give them your email, the site will send notifications when they come across new members that have familial connections with yours.
So if detectives have DNA, they can potentially get a warrant, to compare their DNA sample to the stored samples in Ancestry.com, to see if they can get a familial hit.
There was a long protracted legal battle, on a messed up case like that a few years ago. It turns out that LE got a warrant for the use of the Ancestry.com DNA to compare it to the DNA left behind in a savage child murder. They got a familial 'hit' --they thought so anyway.
And LE looked into the family of the person whose DNA sample at Ancestry gave them a hit. And they began 'investigating' a young man, who was in the same age range and looked similar to the witnesses descriptions of the killer. [ I cannot remember the name of the case--maybe someone else does.]
Anyway, after all that, and all the scrutiny of that individual--it was the wrong guy. The killer was not part of that family. They just hd very similar DNA, maybe distant relatives. So it was a legal mess.
ETA:
https://www.wired.com/2015/10/familial-dna-evidence-turns-innocent-people-into-crime-suspects/
Your Relative’s DNA Could Turn You Into a Suspect
"But the well-publicized success stories obscure the fact that familial DNA searches can generate more noise than signal. “Anyone who knows the science understands that there’s a high rate of false positives,” says Erin Murphy, a New York University law professor and the author of Inside the Cell: The Dark Side of Forensic DNA. The searches, after all, look for DNA profiles that are similar to the perpetrator’s but by no means identical, a scattershot approach that yields many fruitless leads, and for limited benefit. In the United Kingdom, a 2014 study found that just 17 percent of familial DNA searches “resulted in the identification of a relative of the true offender.”
It's not surprising. DNA doesn't lie and with genetic dna, determining the estimated degree of relatedness is a function of the number of centimorgans and matching dna segments. The higher the numbers, the closer the connection. If you are 4th cousins or better, it is very accurate. I've had solid matches that are 5th through 8th cousins.
In terms of how LE can figure out who the relatives of a suspect are, some of the genetic cousins may have posted their family trees on ancestry.com. There is also a "shared" dna function and you can find others who descend from the same line and look at their trees. It can surprisingly easy to figure out the exact connection, even with distant cousins.
There is also a website called Gedmatch.com where you can upload your dna (for free) and then find additional matches. I wouldn't be surprised at all if Gedmatch.com also shares information with LE as well as other sites where dna results are uploaded and cousin matches performed.