Every person has their own distinctive DNA. The only exception is identical twins. DNA is found in the nucleus of all cells in your body except for red blood cells. In criminal cases, typically DNA will be extracted from samples of blood, saliva, hair or skin. In a typical criminal case, a DNA sample is found at the scene of a crime. That sample is sent to the crime lab for analysis. At the crime lab, a chromosone is extracted from the DNA sample. The lab technician will then measure 10 spots on the chromosone and write down the results. This measurement is what makes everyone different, except for identical twins, every person in the world will have different measurements for those 10 spots on the chromosone. An experienced San Francisco Criminal Defense Attorney will hire their own expert to double check the work of the San Francisco Crime Lab.
Now that the police have a measurement for the DNA found at the scene of the crime they will attempt to get DNA samples from any suspects. They can do this by asking for a search warrant from a judge for the DNA sample of any suspect. If the search warrant is granted then the police can forcibly take a blood sample from a suspect. If the DNA from the crime scene match the DNA of the suspect then the police may have solved the crime.
What happens if the police do not have a suspect or the DNA from the suspect does not match the DNA from the crime scene? All convicted felons and those convicted of misdemeanor arson or sex crimes are obligated to give the State of California a sample of their DNA. The police will run the DNA sample from the scene of the crime in the database of DNA samples takern from convicted persons. If they get a match they may have solved the crime.
What happens if they don't get a match from the database? Families will share some of the same DNA, but not all of it. The police can run the DNA sample taken at the scene of the crime through the database and look for only 5 matches instead of the 10 matches that they looked for earlier. If the police get a match for a family member of the person that left DNA at the scene of the crime, they may be on their way to solving the crime.