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.