DNA Evidence Notes
DNA Evidence
Introduction
Terminal Performance Objective: Identify common issues and precautions to correctly obtain and package an elimination DNA sample.
Enabling Performance Objectives:
Identify what DNA is and where it may be recovered during criminal investigations.
Identify how CODIS helps solve crimes.
Identify common causes of DNA sample contamination and precautions to take.
Discuss proper methods of DNA collection from objects and persons.
What is DNA?
DNA can be used to identify (or “fingerprint”) a specific individual.
This can support a successful investigation and prosecution of a suspect, exclude a suspect, or exonerate an innocent person.
DNA Basics
Inherited half from mom and half from dad.
More than 99% of DNA is the same across individuals; forensics focuses on the 1% difference.
Same only in identical siblings.
Stable in dry conditions; does not need refrigeration.
Power of DNA
DNA typing can link evidence to a specific individual with a high level of statistical certainty (except in identical twins).
A person’s DNA is the same in every cell.
DNA collected from a crime scene can link a suspect to evidence or eliminate a suspect (similar to fingerprints).
DNA in a man’s blood is the same as in his skin cells, semen, and saliva.
Victims can be identified through DNA from relatives, even without a body.
Societal Questions
The arrest of Joseph James DeAngelo (alleged Golden State Killer) using genealogical databases raised privacy concerns.
Investigators compared DNA from crime scenes to profiles in a free genealogical database (GEDmatch).
This differs from analyzing DNA of individuals arrested or convicted of crimes, which is stored in the U.S. National DNA Index System (NDIS) since 1989.
The legality and commonality of searching non-forensic databases for law enforcement purposes are under scrutiny.
Investigators are rushing to make similar searches in other cases.
Touch DNA
DNA left behind from skin cells when a person touches an item.
Invisible to the naked eye.
Deposited in smaller amounts than DNA found in bloodstains or other body fluids.
More difficult to identify areas where skin cells may be present.
Challenging to obtain DNA profiles.
Success depends on:
Recognizing suitable items.
Proper collection/storage.
Optimal sampling technique to recover the highest number of skin cells.
Sources of DNA
Forensic examination of a victim or suspect.
Objects found at the crime scene(s).
DNA Evidence in Sexual Assault Cases
Semen from a male suspect on a victim’s body, clothing, or other objects is the most common source of DNA in SA cases.
Blood may also be significant.
Saliva, 2nd most common, can be found:
From kissing, licking, biting, sucking, or oral copulation.
On cigarette butts, cups, soda cans, etc.
On the body or clothing of the victim and suspect.
On objects recovered from the crime scene.
Blood can be transferred from a menstruating female to a male or female.
Especially relevant if scratching occurred or fingers entered the mouth, nose, or eyes.
Other Biological Material
Urine and fecal material can contain cellular material for a DNA profile.
Timeframes for Evidence Collection
Usually 72–120 hours.
Some jurisdictions extend to 168 hours (seven days).
Many jurisdictions have timeframes for medical forensic examination of the victim.
Forensic examination yields usable biological evidence for a limited time.
Based on advances in DNA technology.
How DNA Can Be Used
Place a known individual at a crime scene.
Place an individual in a location they claimed not to be.
Refute a claim of self-defense.
Change a story from an alibi to one of consent.
Second Most Common Source of DNA Evidence
Saliva
EPO #2: CODIS: How it Helps Solve Crimes
CODIS is an electronic database of DNA profiles that can identify suspects, similar to AFIS (Automated Fingerprint Identification System).
Powerful investigative tool.
Every state is implementing a DNA index of individuals convicted of certain crimes (rape, murder, child abuse).
CODIS Uses
Link DNA profiles from a crime scene to a convicted offender.
Provide the investigator with the name of a previously unidentified suspect.
Link an unknown sample to a solved case.
Identify a suspect for the investigator.
Link two or more unsolved cases.
National Missing Person Program
DNA from close relatives can help establish a DNA profile for the missing person.
If samples from children are collected, the spouse or parent in common should also be considered to determine what portion of the child’s DNA is in common with the missing parent.
Samples are sent to an accredited forensic laboratory for DNA testing.
Relatives may voluntarily provide DNA samples.
CODIS and Missing Persons: Sample Removal
DNA profiles are removed when:
The missing person has been identified.
The family member who voluntarily provided the DNA sample is determined not to be related to the missing person.
The family member who voluntarily provided the DNA sample requests in writing that it be removed.
Importance of Other Techniques
DNA is a critical factor in solving crimes, but it’s not the only one.
Other techniques used during criminal investigations are just as important.
There is no one-size-fits-all answer to criminal investigations.
Family Reference Samples
In missing persons cases, samples collected from relatives are known as Family Reference Samples.
DNA Contamination and Precautions
When collecting potential touch DNA items, wear as much PPE as possible to limit contamination via exposed skin, shed hairs, sweat, or saliva.
Greater attention to contamination issues is necessary due to extremely small DNA samples being used as evidence.
Crime scene personnel should avoid speaking over evidence items.
DNA evidence can be contaminated when DNA from another source gets mixed with case DNA.
PCR and Contamination
Because PCR replicates or copies DNA in the evidence sample, the introduction of contaminants or other unintended DNA can be problematic.
PCR will copy whatever DNA is present; it cannot distinguish between a suspect’s DNA and DNA from another source.
Elimination Samples
Like fingerprints, effective use of DNA may require the collection and analysis of elimination samples.
In homicide cases, collect the victim’s DNA from the medical examiner at the autopsy, even if the body is badly decomposed.
Environmental Factors
Biological evidence can be significantly degraded due to environmental factors.
The amount of evidentiary value is typically in inverse proportion to the duration and intensity of exposure to living organisms.
It is recommended that biological evidence be air-dried as soon as possible and kept in paper packaging.
The techniques used to develop DNA profiles are extremely sensitive.
Storage and Preservation
Keep the evidence dry and at room temperature.
Seal, label, and transport in a way that ensures proper ID of where it was found and a proper chain of custody.
Never place evidence that may contain DNA in plastic bags because they retain moisture.
Direct sunlight and warmer conditions may be harmful to DNA.
Avoid keeping evidence in places that may get hot, such as a room or police car without air conditioning.
A very small amount of biological material can produce a usable DNA profile.
Rape Cases and Elimination Samples
It may be necessary to collect and analyze the DNA of the victim’s recent consensual partners to eliminate them as potential contributors.
Proper Methods of DNA Collection
Laboratories test for Touch DNA using either the wet swabbing or cutting methods.
The wet swabbing method is recommended for hard, non-porous items such as metal, glass, or plastic. The surface of the item is rubbed with a wet cotton swab to collect possible skin cells.
The entire object containing the biological evidence should be collected whenever possible.
Not typically feasible for large items of furniture, structures, or other substantial items.
Swabbing Tools
Fresh nitrile gloves
One sterile swab
One swab box or envelope
One outer envelope
Evidence tape
Sharpie marker
The Swabbing Process
The sterile swab will be diluted with sterile or distilled water.
A DNA sample from the object will be taken by placing the tip of the swab in the middle of the stain or object and gently rotating the swab at least one full rotation.
The swab will then be allowed to air dry, and then placed into the cardboard box (or envelope).
The sample will be “double packaged.”
Complete chain of evidence forms.
Collection from a Person: Buccal Swab
A buccal swab (cheek swab) is one of the fastest and most common ways forensic pathologists establish or refute a link between a suspect and a crime scene.
Buccal swabbing is fast, non-invasive, relatively comfortable, and can yield larger sample sizes than other collection methods.
Buccal Swab Kits
Surgical mask
1 sterile swab with cotton, foam, or flocked tips
1 sterile collection box or envelope
Evidence tape
Consent form (if required)
Collecting a Buccal Swab Sample
Anyone can collect a Buccal Swab sample for forensic analysis and doesn’t require a medically trained professional
Do not allow the subject to eat, drink, or use tobacco products for at least 30 minutes before taking the samples.
Wash and sanitize hands, put on gloves
Put on a surgical mask.
Verify the mouth of the subject is empty.
Open the sterile swab and be careful not to allow the swab to touch anything.
Have the subject open their mouth, then place the swab along the inside of the cheek between the cheek and gums and rotate the swab for 5-10 seconds
Using the same swab, repeat on the other side of the mouth.
Immediately remove the swab, being careful not to touch the teeth, lips, or any other surface.
The swab will then be allowed to air dry and then placed into the cardboard box (or envelope).
The sample will be “double packaged.”
Complete chain of evidence forms.
Store sample at room temperature.
Final Note on Evidence Collection
Whenever possible, the entire object containing the biological evidence should be collected.
Summary
This session covered what DNA is and where it may be recovered during criminal investigations.
It also covered how CODIS helps solve crimes, common causes of DNA sample contamination, and precautions to take.
Finally, the session discussed the proper methods of DNA collection from objects and persons.