ELi5: How does DNA tests identify a person if that persons DNA is in no database?

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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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Anonymous 0 Comments

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.

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