© 2015 - Crime Scene Forensics, LLC - All Rights Reserved

What is DNA evidence?


Anything biological...

...blood, semen/sperm, hair,

...tissue, bones, organs, tooth pulp

...bodily secretions, saliva,

...sweat, urine, fingernails

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

There are two types of DNA testing used in criminal and non-criminal investigations:

Mitochondrial DNA:

STR DNA

1. Found in the nucleus of the cell

2. PCR/STR DNA

3. Most commonly used in criminal

    investigation casework

1. Found in the cytoplasm of the cell

2. Runs along the maternal line  

    -  siblings share the same mitochondrial DNA

       with their mother, grandmother, etc...

3. Can be found in hair shafts

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

DNA Collection:

  1. 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

The proper collection and storage of biological evidence for DNA

testing includes the following:


1. Biological evidence should be allowed to air dry before packaging.

    - 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 breathes, and allows the item of evidence to remain dry.

    - Plastic bags do not breathe, and can cause moisture and mold to grow on the   

      evidence, which in turn can have a negative affect on DNA testing

  1. 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  

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

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.  

RFLP (Restriction fragment length polymorphism)

- 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

To cite this website: Crime Scene Forensics, LLC, New York; www.crimescene-forensics.com