What is DNA?
DNA, which stands for deoxyribonucleic acid, is the building block of life. Virtually every cell contains DNA and the DNA in people's blood is the same as the DNA in their saliva, skin tissue, hair, bone, etc. Most importantly, DNA does not change throughout a person's life.
DNA Evidence
DNA evidence, when available, is one of the most important investigative tools because, no two people, unless they are identical twins, have the same DNA. Therefore, DNA evidence can be linked to a suspect or can eliminate a suspect from suspicion. In addition, if no suspect exists, a DNA profile from crime scene evidence can be entered into the FBI's Combined DNA Index System (CODIS) to identify a suspect anywhere in the United States or to link serial crimes to each other.
DNA Evidence Collection
Proper collection of DNA evidence is of the utmost importance. If DNA evidence is not initially identified at the crime scene or on the victim, it may not be collected, or it may become contaminated or degraded.
Since only a few cells are needed for a useful DNA sample, the list below identifies some areas at the crime scene or on the victim that may contain valuable DNA evidence. Remember, even though a stain cannot be seen, there may be enough cells for DNA typing.
| Possible Location of DNA Evidence | Source of DNA |
|---|---|
| Bite mark or licked area | Saliva |
| Fingernail scrapings | Blood or skin cells |
| Inside of outside surface of used condom | Semen, blood or skin cells |
| Blankets, sheets, pillows, or other bed linens | Semen, blood, sweat, hair or saliva |
| Clothing, including undergarments worn during and after the assault | Hair, semen, blood or sweat |
| Hat, bandanna or mask | Sweat, skin cells, hair or saliva |
| Tissue, washcloth, or similar item | Saliva, semen, hair, skin cells or blood |
| Tape or ligature | Skin cells, saliva or hair |
| Crime Scene (Any surface) | Saliva, semen, hair, skin cells or blood |
Given the sensitive nature of DNA evidence, clients should always contact us directly when evidence collection questions arise.
DNA Testing
The most common form of DNA analysis is called polymerase chain reaction (PCR). PCR has allowed investigators to successfully analyze evidence samples of limited quality and quantity. The PCR process makes millions of copies of very small amounts of DNA. This enables the laboratory to generate a DNA profiles, which can be compared to reference DNA profiles In the event of a match, a statistic is generated to reflect how often one would expect to find this particular DNA profile in the general population.
Interpreting DNA Test Results
Three types of results can occur in DNA testing: inclusion, exclusion, and inconclusive.
Inclusion:
When the DNA profile of a victim or suspect is consistent with the DNA profile from the crime scene evidence, the individual is "included" as the possible source of that evidence.
Exclusion:
When the DNA profile from a victim or suspect is inconsistent with the DNA profile generated from the crime scene evidence, the individual is "excluded" as the donor of the evidence.
Inconclusive
Inconclusive results indicate that DNA testing could neither include nor exclude an individual as the source of biological evidence. Inconclusive results can occur for many reasons: for example, the quality or quantity of DNA may be insufficient to produce interpretable results.
DNA Evidence in Unsolved and Postconviction Cases
Advanced DNA technology, such as PCR, makes it possible to obtain conclusive results in cases in which previous testing might have been inconclusive. This can result in the identification of suspects in previously unsolvable cases or the exoneration of those wrongfully convicted. Prosecutors, defense counsel, the court, and law enforcement should confer with us on the need for testing on a case-by-case basis.
