During the investigation of violent crimes such as homicide, fingerprints are not the most common type of evidence recovered by police officers. They are more likely to recover hair or blood. Forensic scientists have been able to use a new technique developed by molecular geneticist Dr. Ray White in 1980. It is popularly called "DNA fingerprinting" and was first admitted as evidence in a court to convict someone in 1987.
In every cell of a person's body, DNA is found in the nucleus. No two people, except identical twins, have the same DNA. DNA is made of a series of 4 bases--adenine, thymine, cytosine and guanine. The order of the bases determines a person's genetic code. Some parts of the code are very similar in all people; for example, the code for building a human heart. However, some portions of the code varies greatly from person to person. These are called polymorphic (poly=many and morphic=shape) segments. Person A might have 6 repeats of a segment, person B may have 17 repeats and person C 12. When several different sequences are considered, the chances that any two individuals will have exactly the same variation are very remote.
To perform DNA fingerprinting, the DNA of cells from a crime scene is copied over and over by a process called polymerase chain reaction (PCR). A single drop of blood or the root of a single hair could be enough to identify the DNA. Using a search warrant, cells are taken from a suspect by drawing blood or taking hair. The DNA from the crime scene and suspect are cut with chemical scissors called restriction enzymes. Then the cut DNA is placed in wells at the top of a gel. The fragments are separated by electrophoresis. If the bands of the suspect's DNA matches the bands of the evidence DNA, the chances are very great that the suspect was present at the crime scene. Interestingly, the first time DNA was used to convict a person in a jury trial in North Carolina was in 1992, in a case of rape that occurred in Catawba County.
Procedure: Case 96-3337