They use DNA to help with crime scences. They do this by finding the DNA a and putting it thought a series tests and it will find a match to a person and that could be the killer.
You can get this D at a crime scene from either under the dead persons nail because there scratching and they get the crimes DNA or maybe there is hair that falls off the criminal that has DNA on it.
Many people have been freed from prison after they were called guilty and some people have been freed from the death sentence because the DNA proved they were innocent.
In paternity testing no man will ever be 100% included as the biological father. But they are very accurate at detecting the criminal from the DNA at the crime scene.