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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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).
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