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DNA Evidence: Basics of Identifying, Gathering and Transporting

Date Published
August 8, 2012

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Gathering DNA Evidence

Physical evidence is any tangible object that can connect an person to a crime scene. Biological evidence, which contains DNA, is a type of physical evidence. However, biological evidence is not always visible to the naked eye. DNA testing has expanded the types of useful biological evidence. All biological evidence found at crime scenes can be subjected to DNA testing. Samples such as feces and vomit can be tested, but may not be routinely accepted by laboratories for testing.

Identifying DNA Evidence

Since only a few cells can be sufficient to obtain useful DNA information to help your case, the list below identifies some common items of evidence that you may need to collect, the possible location of the DNA on the evidence, and the biological source containing the cells. Remember that just because you cannot see a stain does not mean there are not enough cells for DNA typing. Further, DNA does more than just identify the source of the sample; it can place a known individual at a crime scene, in a home, or in a room where the suspect claimed not to have been. It can refute a claim of self-defense and put a weapon in the suspect's hand. It can change a story from an alibi to one of consent. The more officers know how to use DNA, the more powerful a tool it becomes

Types of Evidence, Possible Locations, and Sources of DNA
Evidence Possible Location of DNA on the Evidence Source of DNA
baseball bat or similar weapon handle, end sweat, skin, blood, tissue
hat, bandanna, or mask inside sweat, hair, dandruff
eyeglasses nose or ear pieces, lens sweat, skin
facial tissue, cotton swab surface area mucus, blood, sweat, semen, ear wax
dirty laundry surface area blood, sweat, semen
toothpick tips saliva
used cigarette cigarette butt saliva
stamp or envelope licked area saliva
tape or ligature inside/outside surface skin, sweat
bottle, can, or glass sides, mouthpiece saliva, sweat
used condom inside/outside surface semen, vaginal or rectal cells
blanket, pillow, sheet surface area sweat, hair, semen, urine, saliva
"through and through" bullet outside surface blood, tissue
bite mark person's skin or clothing saliva
fingernail, partial fingernail scrapings blood, sweat, tissue

Crime Scene Integrity

Protection of the crime scene is essential to the protection of evidence. Safeguarding and preserving evidence is fundamental to the successful solution of a crime. Remember, while documenting evidence at the crime scene, to include descriptions of whether evidence was found wet or dry. An example of this documentation would include blood spatters.


The risk of contamination of any crime scene can be reduced by limiting incidental activity. It is important for all law enforcement personnel at the crime scene to make a conscious effort to refrain from smoking, eating, drinking, littering or any other actions which could compromise the crime scene. Because DNA evidence is more sensitive than other types of evidence, law enforcement personnel should be especially aware of their actions at the scene to prevent inadvertent contamination of evidence.

Chain of Custody

The chain of custody of evidence is a record of individuals who have had physical possession of the evidence. Documentation is critical to maintaining the integrity of the chain of custody. Maintaining the chain of custody is vital for any type of evidence. In addition, if laboratory analysis reveals that DNA evidence was contaminated, it may be necessary to identify persons who have handled that evidence.

In processing the evidence, the fewer people handling the evidence, the better. There is less chance of contamination and a shorter chain of custody for court admissibility hearings.

Transportation and Storage

The first responding officer may be called upon to transport evidence from a crime scene. As with any evidence, the officer should ensure that the chain of custody is maintained. In addition, they should be aware that direct sunlight and warmer conditions may degrade DNA, and avoid storing evidence in places that may get hot, such as the trunk of the police car. To best preserve DNA evidence, store in a cold environment.

Any probative biological sample that has been stored dry or frozen, regardless of age, may be considered for DNA analysis. Nuclear DNA from blood and semen stains more than 20 years old has been successfully amplified (copied) using polymerase chain reaction (PCR) and subsequently analyzed. Samples that have been stored wet for an extended period of time may be too degraded for DNA analysis and should be checked using PCR. Analyzable mitochondrial DNA has been recovered from very old bones, teeth, and hair samples.

Samples generally considered unsuitable for testing with current techniques include embalmed bodies (with the possible exception of bone or plucked hairs), pathology or fetal tissue samples that have been immersed in formaldehyde or formalin for more than a few hours (with the notable exception of pathology paraffin blocks and slides ), and urine stains. Other samples such as feces, fecal stains, and vomit can potentially be tested, but are not routinely accepted by most laboratories for testing.

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Date Published: August 8, 2012