DNA stands for deoxyribonucleic acid. It is the genetic material found in nearly every cell in the human body. It is as unique to each person as a fingerprint, and can be used to exclude a suspect in a crime, or indicate that the person is guilty.
DNA can be found at a crime scene through many things; bodily fluids, hair, skin cells. It can be obtained from almost any source, a hat, a drinking glass, a hair left at the scene. Since it is found in virtually all human cells, any cell left behind at the crime scene that is found by the police, can be examined.
Forensic scientists speak in their own language. A suspect can be either excluded or not excluded. If the person cannot be excluded, then the scientist usually says there is an astronomically high probability it is someone beside you. In other words, the scientist might say something like there is a one in one trillion chance the DNA belonged to someone OTHER than you. For all practical purposes this means the DNA belongs to you.
What if there is no DNA left behind at the crime scene? That would neither help nor hurt. Since there isn’t any DNA pointing to someone else, it is simply not considered.
But what if there is DNA found at the scene, but it excludes the suspect? The answer depends on where the DNA is found and the totality of the circumstances. Let’s say there is a crime where the perpetrator was seen wearing a hat. And the hat was left behind at the scene. The hat is analyzed and DNA is found on the hat but it excludes the defendant in the case. You would think this would be the end of it and the defendant would be released. Not so fast. The scientist will testify that it is possible to wear the hat and not leave behind any DNA. Therefore, the defendant could have borrowed the hat from someone else (which would explain the DNA belonging to someone else), worn the hat at the crime and simply not left behind any DNA on the hat. In order to win the case, the defense lawyer might have to provide additional information. DNA is only one part of the evidence presented in court and does not necessarily determine the outcome of the case by itself.