DNA in forensics

1. How is DNA Used in
Forensics?

2. What is forensic science
Forensic science is the scientific
method of gathering and
examining information about the
crimes and criminals.

3. DNA Collection & Comparison
• Overview:
– Investigators gather samples from the crime
scene and from suspects and then analyze it
for a set of specific DNA regions or markers.
– A match of one marker is not usually unique,
but if a sample matches four or five markers,
there is a very good chance it is a match.

4. DNA Collection & Comparison
• DNA is collected at crime scenes in a
variety of ways using tools such as:
– Smear slides, scalpels, tweezers, scissors,
sterile cloth squares, UV light, luminol and/or
blood collection kits (for sample collection of
suspects or living victims)
• DNA samples can be from:
– Saliva, blood, hair strands, skin, finger or toe
nails, and/or a tooth with root material

5. DNA Collection & Comparison
• How is blood collected?
– Blood on Clothing?
• Investigators submit whole pieces of clothing or
they may use a sterile cloth square and a small
amount of distilled water
– Dried blood on furniture?
• Investigators send the whole object to the lab
– Dried blood on a wall, tub or some other
object too big or difficult to move to the lab?
• Investigators scrape the blood sample into a sterile
container for further analysis

6. DNA Collection & Comparison
• Analysis of DNA??
– Uses various DNA Technologies
• RFLP
• PCR
• STR
• Mitochondrial DNA Analysis

7. RFLP: Restriction Fragment Length
Polymorphism
• Analyzes variable lengths of DNA
fragments
• One of the original applications of DNA
analysis
• Not used as much anymore because it
requires a large quantity of DNA sample
and samples degraded by the environment
do not work well with RFLP

8. PCR: Polymerase Chain Reaction
• Used to make millions of exact copies of
DNA from a biological sample
• Allows very small samples to be analyzed,
such as a sample of a few skin cells
• Must be very careful about contamination
in this process

9. STR: Short Tandem Repeat
• Evaluates specific regions (loci) within
nuclear DNA
• FBI uses 13 standard specific STR regions
for CODIS

10. Mitochondrial DNA Analysis
• Used for samples that cannot be analyzed
using RFLP or STR
• Uses DNA extracted from mitochondrion
rather than nuclear DNA
• Especially useful in old cases and old
samples

11. DNA Collection & Comparison
• What happens after the samples are collected?
– A DNA profile is created….how??
• Markers are found by designing small pieces of DNA
(probes) that will seek out and bind to complementary DNA
sequences. This creates a distinct pattern. Again, one
marker is not usually unique, but with four or five regions the
match is likely
– The DNA profiles are compared with samples from
suspects to find possible matches.
– If there are no suspects, a national database called
CODIS may be used to find potential suspects.

12. DNA Collection & Comparison
• More on CODIS:
– Stands for Combined DNA Index System
– National Network that helps identify leads for
crimes with no suspects
– Three tiers: Local (LDIS), State (SDIS),
National (NDIS)
– Uses 13 DNA regions that vary from person to
person
– Looks for matches at more than one location
on a genome for more accurate results

13. Sources of DNA at Crime Scenes
• Cool table at:
– http://www.dna.gov/basics/evidence_collectio
n/identifying

14. Sources of DNA at Crime Scenes
• Examples of sources from real cases:
– Saliva on the stamp of a stalker’s threatening letter
– Skin cells shed on a ligature of a strangled victim
– Perspiration on a baseball cap discarded by a rapist
was compared with the DNA in the saliva swabbed
from a bite mark on a different rape victim
– DNA analysis of a single hair (without the root) found
deep in a victim’s throat
– Maggots can contain DNA
of a perpetrator

15. How can DNA evidence be
planted??
• Sneezing or coughing over evidence
• Person touches their mouth, nose or other
part of the face and then touches the area
that may contain the DNA to be tested.
• Scene personnel can deposit hairs, fibers,
or trace material from their clothing
• Wind can carry in contaminants

16. Other Uses of DNA
• Paternity Testing and Proving Family
Relations
• Identification of John or Jane Does
• Study of evolution and ancestry
• Studying Inherited Disorders

17. Other Uses of DNA
• Identifying Stolen Trees, Poached Animals

18. References
• http://www.safenetwork.org/DNA_Information.ht
ml
• http://www.troopers.state.ny.us/Forensic%5FSci
ence/DNA/
• http://www.ornl.gov/sci/techresources/Human_G
enome/elsi/forensics.shtml
• http://www.howstuffworks.com/dna-
evidence3.htm
• http://www.howstuffworks.com/csi4.htm
• www.ncjrs.gov/txtfiles1/nij/bc000614.txt

19. References
• www.cji.net/CJI/CenterInfo/fscec/Contamination.htm
• http://www.courttv.com/trials/westerfield/071702_ctv.html

20. THANK YOU