Say a crime has been committed, and the police find either blood or semen at the crime scene. However, the perpetrator has committed no prior crimes and therefore his DNA is in no database or registry. How are analyst able to find out who the perpetrator is through analyzing either the blood or semen found at the scene?
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If the DNA doesn’t match, then it’s not someone in the database. However, there are other sources of DNA matching – like 23andMe. From that the police might find a relative, and then they know it’s someone in a particular family. From there it’s plain old police work to see if they can prove who did it.
Most likely, someone in their close family has DNA in the system, either from a previous encounter with the law or from submitting to a DNA test from a company that shares information with law enforcement. Say your father took a test and the police have your DNA sample. They see that he is a 50% match, so they have limited their likely suspects to his parents, siblings and children already.
On a related note, don’t do a 23andMe or similar. They have been shown to give inaccurate results and then everyone related to you is traceable.
At this point we have tons and tons and tons of genetic profiles saved to databases, both public and private. For example, various law enforcement groups keeps databases from their records AND they have access to things like 23&Me and other private companies’ records.
Even if they don’t have *your* DNA, they might have your cousin, or your Mom or your half-brother, whatever. It becomes a sort of Periodic Table of Element system, they don’t know you exist. But they can tell that a Man, approximately 35 years old, who has cousin X and grandfather Y, was in Terre Haute on October 17th, etc. Again, they don’t know who you are, but they posit you exist and when they find you, it’s pretty hard for you to argue that all those variables can link up and point to a different person.
DNA evidence isn’t used that way. They don’t just find the culprit from the DNA found at a scene.
Instead DNA evidence would typically used to corroborate other evidence against someone who is already a suspect. Like someone would be found to have a motive to kill the victim, be spotted in the area the victim was found, the murder weapon being found in their car, *and* their DNA evidence being found at the scene. Under such circumstances the investigators already know who they think did it and can just compare their DNA, they aren’t just finding DNA and using it to identify the culprit from the population at large. In concept it might be possible to get general features like the DNA being from a male, Caucasian, with brown hair, but not much more than that.
Usually it is used the other way round. You first find a suspect and then check their DNA against the DNA you found at the crime scene.
This works because most crimes like murder and similar are not actually committed by complete strangers. Most murders are not a whodunit mystery and more of a how to prove it thing.
Of course you can check the DNA of any sample you find against a database of known DNA, but in most countries that will be rather small and limited to people with criminal convictions and sometimes government employees.
The whole idea of finding a sample and then figuring out who it belongs to because you have everyone’s DNA on file used to by a thing for dystopian sci-fi only and has only recently become more of a thing in real life.
Very unlikely, but not impossible. It possible to determine how closely related someone is to another using DNA comparisons and it is possible to obtain DNA from suspected criminals if the police have cause to suspect them.
As others have the short version is that it doesn’t.
What DNA does do is allow the police to specifically place a known suspect at the scene, after other angles of investigation creates reason to think they might be the person they were after. So, its more a end point of a line of investigation than a start point.
A big part of the reason is that DNA is not truly unique, at least in the manner we can test. In a big city like new York, thier might, in theory, be a dozen people who might “match” the DNA sample, so you’d need more that “just” DNA to convict them (for example, a know motive or association with the victim, proof that only one of those people could have had access to the location, etc).
DNA is linked to family members. You can conclude with a very high level of confidence that certain people are related to others by comparing DNA between people. So if they have family members that are in any database, like 23andme and not just a LE database, they can match on a certain % of similarity. Then LE goes down that path to see what becomes of it because it drastically narrows the field.
There’s different types of testing. Commonly this happens when they pool databases of tested profiles. Your parents, siblings, aunts/uncles not married into the family and cousins are used as references. They can given enough supplemental data say that the unknown profile is related and fits into x position of a family tree.
Others have answered the question in general but I wanted to add a bit more info.
Specifically about using databases such as 23andme or Ancestry. These two databases are closed databases. They require specific types of court orders if the police want to use them. And it would need to be requesting a specific person or people. Basically they need to identify a suspect first and have phenomenally good legal justification to make that request. And in general the individual is informed by the company so they can challenge the request in court if they wish.
In practice it’s usually not done as there are better and easier and better ways to get the information. For example 23andme have had a grand total of 11 requests for 15 individuals data since 2015. All within the USA; none from anywhere else in the world. And, based on the wording of their transparency report, I’m not even sure if the data was released.
There are open databases like GEDmatch or MyFamilyDNA that give more broad access to the public. And it is these kind of databases that are used in the type of searches that police would use, including forensic genealogy which is looking for relatives of those of the dna sample individual (eg the Green River Killer, Gary Ridgeway, was eventually identified through forensic genealogy)
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