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DNA
Types of evidence and how they
are tested for DNA
2009©Forensic Science Today
Alec Jeffries
Asked to determine paternity
Child wanting to immigrate
Was he related?
See also Audri Kowalyk, Introductory
Forensic Science Educational Activities,
Unit 7 History is Made: First Crime Solved
Using DNA
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First Criminal Case
DNA first used in Colin Pitchfork case,
1988.
3 villages were “sampled”, approx 5000
men.
Also exonerated Richard Buckland.
(See handout from Kowalyk, History Is Made. .
Lynda Mann/Dawn Ashcroft rape & murder case)
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Examples
Anastasia and Tsar Nicholas
Egyptian Mummies
Paternity
Larry Hillblom
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Courier company DHL
Young girls
Children
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Deoxyribonucleic Acid
Genetic make-up for every person is
unique.
Inherit half from your mother and half from
your father.
Half can be any combination of that
parent’s alleles.
An allele is a single coding on a location
on your DNA
A gene is made of a pair of alleles
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DNA – The Basics
First discovered in 1868
1950’s- James Watson & Francis Crick deduced
the structure of DNA
DNA is a polymer composed of repeating units
called nucleotides.
Nucleotide is made up of a sugar molecule, a
phosphorus-containing group and a nitrogencontaining base.
Bases are adenine, thymine, guanine, cytosine
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DNA Structure
http://nobelprize.org/educational/medicine/dna_double_helix/readmo
re.html
James Watson – Left
Francis Crick - Right
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Complementary Base Pairing
A - T and G – C base pairs only
No restrictions on sequencing
Average human chromosome has 100
million base pairs
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What is DNA Found In?
Found in these parts of your body
materials
Blood
Saliva
Skin cells
Sperm
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DNA Can Be Found On..
Can be left in the following types of evidence
Anything that rubs against skin
laundry, hats, bed clothes, glasses
Anything that you breathe on
bandana, mask, tissue
Anything you exchange saliva with
toothpick, cigarette, drink container, stamp, envelope,
toothbrush, bite mark etc.
Anything you “exchange” skin cells with
other person’s fingernails, watch bands, tape stuck on you,
fecal matter
Anything you exchange body fluids with
condoms, bullets that travel through the body
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Where is DNA Found in the Cell?
The nucleus
In the form of chromosomes
A very long DNA molecule containing the nucleotide
sequence (genetic code)
Inherited from both parents
Humans have 46 chromosomes (23 pairs)
Sex chromosomes (XX in females, XY in males)
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Where is DNA Found in the Cell?
The mitochondria
Smaller than nuclear DNA
Inherited from a person’s mother
Mitochondrial DNA produces enzymatic proteins
that function to convert energy into a usable
form within the cell.
mtDNA is constructed in a circular or loop
configuration.
See Figure 13-17, p. 388, p. 389 Law Enforcement Bulletin
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Nuclear vs. Mitochondrial DNA in
Forensic Identification
Nuclear DNA is more informative
Cell contains one nucleus but thousands of
mitochondria.
Mitochondrial DNA lasts longer and is more
easily extracted from degraded samples such as
ancient bones.
A mtDNA reference sample can be obtained
from any maternally related relative
Forensic analysis of mtDNA is more rigorous,
time consuming and costly than nuclear DNA
profiling
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Nuclear DNA
Nuclear DNA has been used
to determine the descendants of
Thomas Jefferson
Nuclear DNA determines
paternity in child custody cases
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Nuclear DNA Findings
Nuclear DNA has been used to analyze
the genetic qualities of Egyptian mummies
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Discoveries from Mitochondrial
DNA
Used to test charred remains of Tsar
Nicholas and his family who were
executed during the Russian revolution of
the early 20th century
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Steps Involved in the Examination
of Evidence for DNA- Extraction
Possible sources of DNA are body tissue,
organs, root of hair, bodily fluids or bone
Chain of custody must be maintained!
Each sample must be handled with a
clean pair of gloves or disposable forceps.
Samples should be kept cold and stored in
a freezer.
Wet or liquid samples should be air-dried.
2009©Forensic Science Today
Steps Involved in the Examination of
Evidence for DNA-Analysis by RFLP
Restriction Fragment Length Polymorphisms
 First accepted analytical method for DNA used in the
U.S.
 Must have nickel sized sample to test
 Tests for patterns that occur by cutting the DNA using
specific enzymes. When the cut is made, the sections
are separated by their size, using electrophoresis. (See
pp. 135-136, Figure 13-9, p. 373)
 Can give results that are relatively unique
 Can not use degraded DNA
 Used in the impeachment trial of President Bill Clinton to
identify the semen stain on Monica Lewinsky’s dress.
See p. 374
 Technology has been phased out. Replaced by PCR.
2009©Forensic Science Today
•
Contributed by Harold Messler, Manager of the St. Louis Metropolitan
Police Department Crime Lab
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Steps Involved in the Examination
of Evidence for DNA- Amplification
Use of Polymerase Chain Reaction (PCR) to copy a
small sample of DNA. Sample may be degraded.
Prevention of false results requires carefully applied
controls and technique.
Cross contamination with known DNA must be avoided.
Sample is placed in a thermal cycler with a mixture of
nucleotides and DNA replication enzyme (DNA
polymerase) See p. 367 Figure 13-5
High temperatures (94ºC) cause the DNA helix to
separate. (denaturation)
DNA copies itself using the nucleotides and DNA
polymerase. (See pp. 372-376)
Separation & duplication process is repeated 28-30
times.
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DQ-alpha PCR
Contributed by Harold Messler, Manager of the St. Louis Metropolitan
Police Department Crime Lab
2009©Forensic Science Today
Steps Involved in the Examination of
Evidence for DNA-Comparison of STR
Markers*
 Samples as small as a few cells
 Works for very degraded samples
 Uses PCR technique to amplify sample
 Looks at 13 specific STR regions (p. 379 Table 13-1)
 An example of an STR is D7S280, it is found on human
chromosome 7. The sequence GATA is repeated within
this loci.
 Different individuals have different numbers of repeats at
these loci
 Used to create DNA profiles for databases-a dozen or
more matching STR markers constitutes a match
2009©Forensic Science Today
Image provided by Promega Corporation. See www.promega.com for more information.
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Single Nucleotide Polymorphisms
SNPs
An alteration in a gene that occurs in at
least 1% of the population.
Need at least 50 SNPs to give same
results as 13 STR sites.
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SNP Example
SNPs, typed for 70 non-sex genetic
markers
Go to the following website for an example
of an SNP and information about them.
http://www.dna.gov/research/alternative_markers/autosomalsnps
2009©Forensic Science Today
DNA Results
Must be explained in relation to statistical
probability.
Since the entire DNA is never tested, DNA
technicians should not say that the results
can match only one person.
Instead, they explain the probability of
another person matching these results.
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Protect evidence
Prevent damage before collection
Do not move unless it is a high risk of loss.
If you must move the evidence, carefully
document its location and the action you
are taking.
Note location and description of evidence
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Collection of DNA Evidence
Change gloves between each piece of
evidence collected to prevent crosscontamination
Do not collect in plastic. If not dry, molding
can occur which will degrade the DNA
Collect in separate containers, properly
labeled and sealed
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Collection continued
Sexual assault evidence on victim

Protect the evidence by loosely wrapping
victim in clean sheet while transporting victim
to hospital for sexual assault examination
Always collect evidence in paper
containers unless the evidence can tear
the container
Air-dry evidence before collection, when
possible.
2009©Forensic Science Today
Protecting DNA
Must be kept cold, -80oF
Must not expose it to your DNA by
touching it (skin to sample) or breathing,
coughing or sneezing on it.
2009©Forensic Science Today
Types of DNA Databases
Sex Offender
Felony Offender
Felony Arrest
All Type Arrest
Total 1,407,000 DNA Profiles in
the U.S. CODIS database and
64,500 in the forensic
database.
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CODIS
Combined DNA Index System
National database of DNA STR test results
handled by the FBI
Collected from crime scene samples,
convicted persons.
Contains samples from solved and
unsolved crimes
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Offender Hits
at State DNA Index System (SDIS)
Through May 2004
8,949 Offender Hits in 42 States and 2 Federal Laboratories
96
238
35
2
8
90
2
329
4
1
1
30
11
508
83
44
161
75
337 187
47
24
239
1715
75
113 (MA)
1516
340
7
50 (CT)
18 (NJ)
54 (MD)
20 (FBI lab)
47
37
33
295
38
266
270
2 (US Army)
122
20
1360
Hawaii
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Puerto Rico