|DNA Evidence can be found virtually anywhere
- If it's biological, it's potential DNA evidence
- People constantly shed DNA
- There are 3,000,000,000,000 cells in the human body...only
about 100 are needed to obtain a DNA profile
|What is DNA evidence?
...blood, semen/sperm, hair,
...tissue, bones, organs, tooth pulp
...bodily secretions, saliva,
...sweat, urine, fingernails
|There are two types of DNA testing used in criminal and non-criminal investigations:
|Uses of DNA in Criminal Investigations:
1. Identifying remains
2. Provides a Record of a particular person
being in a particular place
3. Ownership of personal items such as clothing
4. Determining Kinship
|1. STR DNA increased sensitivity in testing
2. STR DNA increased awareness in evidence handling
- As a result, the crime scene investigator must take great precautions to avoid
the contamination of DNA evidence
In the past, all crime scenes needed to be protected against elements such as: Weather, Animals,
and Traffic. With the inception of STR DNA testing, the crime scene investigator must also be
concerned with scene contamination by crime scene personnel.
It is imperative that crime scene personnel take precautions to avoid contamination of DNA evidence:
1. Personal Protective Equipment (PPE) should be worn at all times.
- a single hair or drop of sweat from an unprotected person could leave an unknown DNA
sample at the scene
2. When collecting DNA samples, the crime scene investigator should change gloves and forceps
after each item is collected, in order to avoid cross contamination between items of evidence
1. Photograph stain/sample with and without a scale
2. Carefully swab the stain
- preferably with no dilution
- if stain is dry, use one drop of distilled water on the swab
3. Allow to air dry before packaging
4. Carefully label & seal (date, time, initials).
5. Maintain the chain of custody.
6. Provide information that you think may be important.
- Note if items were collected from an unusual location or
might have possible contaminants.
7. Forward to the forensic lab ASAP
The proper collection and storage of biological evidence for DNA
testing includes the following:
1. Biological evidence should be allowed to air dry before
- ideally, it should be hung up in a clean dry room,
away from direct sunlight
2. Biological evidence should be packaged in paper bags.
- paper breaths, and allows the item of evidence to
- Plastic bags do not breath, and can cause moisture
and mold to grow on the evidence, which in turn can
have a negative affect on DNA testing
3. Biological evidence should be stored under laboratory
conditions as available resources permit - or in a
cool, dry climate, free of moisture
4. Place liquid items in collection tubes and refrigerate
Before a DNA test can be performed, a control sample should be
obtained from the victim, suspect, and any other persons whose DNA
may be found on the item of evidence.
A control sample can be in the form of whole blood, a buccal swab, or
any other known exemplar from the person in question. The easiest
type of DNA control sample to obtain is a buccal swab. A buccal swab
involves swabbing the inside of a person's cheek with an approved
type of swab, for about 30 seconds, to secure the buccal cells that are
found on the inside of the cheek. If done properly, a full DNA profile
can be obtained from this swab.
–Before obtaining DNA control samples, consider having the person
in question sign a DNA consent form.
|- this type of test usually provided a fairly discriminating result
- IE: a probability result of 1 in 1,000,000
PCR (Polymearse Chain Reaction) - first generation
- to run this type of test, a very small biological sample is needed, and that sample
can be amplified, or reproduced, to obtain a DNA profile
- the results are not as discriminating
- IE: a probability result of 1 in 25,000
STR (Short Tandem Repeat) second generation PCR
- This is the current form of DNA testing currently used in most forensic DNA labs.
STR testing provides the “best of both worlds”
- only a very small biological sample is required, (sometimes the sample is not even
visible to the naked eye.)
- STR DNA provides a very discriminating result
- IE: a probability result of 1 in 300,000,000,000