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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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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