FORENSIC DNA TESTING
In many criminal cases, blood typing is the primary evidence. Juries have sometimes convicted suspects on ABO typing combined with other blood antigens giving overall probabilities as low as 25% to 50% that the suspect and the blood evidence matched. DNA evidence can do much better and provide probabilities that are almost 100%.
No two individuals have the same DNA. During gamete development and fertilization, sets of individual chromosomes are distributed to offspring in so many possible combinations that it is incredibly unlikely that any two individuals will have the same DNA. Identical twins are the exception that proves the rule, because they occur when the egg divides after fertilization has already happened.
DNA tests alone, without supportive evidence, have sometimes been sufficient for conviction, where identity was the key issue. DNA evidence is now almost always sufficient for exoneration of misidentified individuals who were wrongly convicted of committing a crime. DNA evidence can be obtained from any body tissue or secretion that has cell nuclei that contain DNA. There are two major types of testing used to determine whether DNA found at the scene of a crime matches that of the suspect or the victim. One is popularly known as DNA fingerprinting , and the other is polymerase chain reaction (PCR) amplification followedby hybridization or sequencing. Tissue samples taken from suspects may be compared with evidence obtained from a crime scene.