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Introduction to Postconviction DNA Testing Innocence Network Conference April 19, 2011 – Charlotte, North Carolina. Olga Akselrod Senior Staff Attorney The Innocence Project. Seth Miller Executive Director Innocence Project of Florida. DNA Basics – What Is DNA?.

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Introduction toPostconviction DNA TestingInnocence Network ConferenceApril 19, 2011 – Charlotte, North Carolina

Olga Akselrod

Senior Staff Attorney

The Innocence Project

Seth Miller

Executive Director

Innocence Project of Florida

dna basics what is dna
DNA Basics – What Is DNA?
  • Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms
  • Two kinds of DNA:
    • Nuclear
    • Mitochondrial
dna basics what is dna1
DNA Basics – What Is DNA?
  • Nucleas = 46 Chromosomes
    • 23 from egg (mother)
    • 23 from sperm (father)
  • Many Mitochondria
nuclear dna testing

Target Region for PCR

Nuclear DNA Testing

What genes are we looking at?

Base Pair mutations in highly variable regions in the DNA

Base pairs: adenine (A) forms a base pair with thymine (T) and guanine (G) forms a base pair with cytosine (C)

We test these regions because they vary greatly from person to person, not because they have a specific genetic purpose


nuclear dna testing1
Nuclear DNA Testing

DNA testing kits test at 13-16 regions (aka “markers” or “loci”)




Image :

to get a dna profile just count the number of repeats
To Get A DNA Profile, Just Count The Number of Repeats

At each locus, you have genetic information inherited from your mom and your dad in the form of a number of repeats. The number of repeats are called “alleles”.

D8 14, 14

D21 29, 31

D7 8, 11

CSF 7, 10

D3 15, 15

THO1 8, 9

D13 11, 11

D16 9, 13

D2 21, 22

D19 12, 12

vWA 14, 16

TPOX 9, 9

D18 10, 15

Amel X, Y

D5 12, 12

FGA 20, 23

analyzing crime scene evidence
Analyzing Crime Scene Evidence
  • Extraction
  • Quantification
  • Amplification
  • Electrophoresis
nuclear dna testing statistics
Nuclear DNA Testing - Statistics

The probability of a random match is calculated by applying the product rule to the probability of the allele found at each locus

0.195 x 0.075 = 0.0146 = 1 in 68 people (1/.0146)

0.195 x 0.075 x 0.036 = 0.0005 = 1 in 2000 people (1/.0005)


Multiply them ALL together and you get 1 in 594 trillion!

Modified from

old dna testing methods
Old DNA Testing Methods
  • RFLP: late 1980s-early 1990s
    • Very discriminating
    • Not very sensitive
  • DQ Alpha: early 1990s-mid/late 1990s
    • More sensitive than RFLP, but less discriminating
      • More sensitive because was first method to use PCR
    • Same basic principle as current testing but different markers
    • DQ Alpha Polymarker – Additional polymarker enzymes added to make the test more discriminating
current dna testing methods
Current DNA Testing Methods
  • STR (Short Tandem Repeat): late 1990s-present
    • Advantages
      • Much more sensitive than older methods
      • Number of markers makes it highly discriminating
      • Compatible with CODIS
      • Allows for sex typing
      • (Differential extraction was also developed around the same time)
current dna testing methods1
Current DNA Testing Methods
  • Y-STR (Y Chromosome DNA Testing): early 2000s-present
    • Variant of autosomal STR that tests for DNA in the Y chromosome
    • Advantages
      • Good for use in samples with little male DNA and lots of female DNA
      • Good for use in samples with DNA from multiple males
      • Paternally inherited so you can use relatives’ DNA as elimination samples
    • Disadvantages
      • Paternally inherited so it’s not as discriminating as STR
      • Not compatible with CODIS
      • Can’t be used where paternal relative or female is alternative suspect
current dna testing methods2
Current DNA Testing Methods
  • MiniFiler STR: late 2000s – present
    • Advantages
      • Good for degraded samples
      • More sensitive so less DNA needed
      • Good as adjunct to STR
        • Where STR only gets partial profile, miniSTR may help to obtain a more complete profile
    • Disadvantages
      • Because it is more sensitive, can produce complex mixed profiles, including contaminants
  • Other Newer Kits That May Be Good for Degraded/Inhibited Samples
    • Powerplex Hot Start (HS) 16
    • Identifiler Plus
current dna testing methods3
Current DNA Testing Methods
  • Mitochondrial DNA testing: late 1990s-present
    • Advantages
      • Good for testing hairs without roots
      • Maternally inherited so you can use relatives’ DNA as elimination samples
    • Disadvantages
      • Maternally inherited so less discriminating
      • Not compatible with CODIS or state databases
      • Mixtures are extremely difficult to interpret
strategies for using dna to exonerate
Strategies For Using DNA To Exonerate
  • Three main ways that DNA can be used to prove innocence:
    • Exclusion of defendant from probative item
    • Exclusion plus getting redundancy
    • Exclusion plus identifying contributor of DNA
strategies for using dna to exonerate1
Strategies For Using DNA To Exonerate
  • Strategy #1 – Exclusion of the defendant from DNA on a probative item
    • Exclusion may be sufficient if there is evidence that biological material you want to test came from the perpetrator, i.e.:
      • Intimate samples – semen or foreign pubic hair from rape kit
      • Cigarette that witness saw perpetrator smoke
      • Drink or food that witness saw perpetrator drinking or eating
strategies for using dna to exonerate2
Strategies For Using DNA To Exonerate
  • Strategy #1 – Exclusion of the defendant from DNA on probative item (cont.)
    • Could someone besides the perpetrator have deposited the biological evidence?
      • Often exclusion alone isn’t sufficient because there are alternative explanations for the source of the biological material
        • Blood at crime scene where struggle occurred
        • Hair on victim or near victim
        • DNA in fingernail scrapings/clippings
        • Sweat and skin cells on clothing worn by perpetrator
      • Even in rape cases where testing semen, you need to exclude the other possibilities, such as consensual sex partners
      • If it isn’t an intimate sample or the DNA cannot conclusively be attributed to the perpetrator, you will likely need more than just an exclusion
strategies for using dna to exonerate3
Strategies For Using DNA To Exonerate
  • Strategy #2 - Exclusion plus getting redundancy
  • If uncertain that biological material from a single item comes from the perpetrator, a redundancy to other biological material connected to crime can build a stronger case
    • Get the same person’s profile on multiple probative items i.e., perpetrator’s clothing, ligature, and blood on floor; or
    • Multiple crime scenes exhibit same DNA profile with strong MO
  • May also be important where defendant was excluded at trial
    • Prosecution argues DNA not from perpetrator, but redundancy makes that argument less plausible
  • Note that you can’t compare DNA test results from different methods (i.e. a Mito result can’t be compared with an STR result), so if you need a redundancy to exonerate, must be careful in selecting methods to use.
strategies for using dna to exonerate4
Strategies For Using DNA To Exonerate
  • Strategy #3 – Excluding plus identifying actual perpetrator
    • Two ways
      • CODIS– state and federal DNA databases
      • Compare DNA from alternative suspect
    • May also be a good strategy where defendant already excluded pretrial through serology or early DNA but was still convicted
      • i.e. Doug Warney and Jeff Deskovic
strategies for using dna to exonerate5
Strategies For Using DNA To Exonerate
  • Even if the DNA test results are unable to prove actual innocence, they may demonstrate a right to a new trial by showing attribution was incorrect, for example:
    • DNA testing disproves serology inclusion of blood on defendant’s clothes attributed to victim
    • DNA testing disproves hair microscopy inclusion
post conviction dna testing is still a good call even though
Post-conviction DNA Testing Is Still A Good Call Even Though . . .
  • Hair microscopy connects defendant to the crime
  • Serology included the defendant
    • How many enzymes?
  • Lack of results pretrial
    • What methods did the lab use?
    • Could they have missed biological material?
    • Could they have not used enough sample?
  • Partial DNA match
    • What is the inclusion statistic?
    • Did they use an early kit with few markers?
postconviction dna testing is a good call when
Postconviction DNA Testing Is A Good Call When . . .
  • Some factual scenarios make finding DNA MORE likely
    • Victim says perpetrator ejaculated or semen/sperm found pretrial; nothing about condom
    • Earlier results detected foreign antigens/alleles
    • Nature of the crime
      • Try to reconstruct the crime from testimony, photos, reports etc. to help determine whether perpetrator would have left behind DNA
      • Signs of struggle = increased chance that perp was injured
        • Defensive wounds
        • Overturned furniture; general disarray
      • Strangulation makes DNA in fingernail scrapings more likely
      • Violent stabbing makes finding perpetrator blood more likely
      • Hot weather or level of struggle can make perpetrator sweat and skin cells more likely on perpetrator clothes left at crime scene
      • Other logical inferences
dna testing won t always make sense
DNA Testing Won’t Always Make Sense
    • DNA helps us establish identity, so if identity isn’t at issue, it probably isn’t a DNA case
    • Consent defense
    • Self defense
    • Other fabrication cases
  • Biological material lacks strong connection to the crime
dna testing is not a panacea
DNA Testing Is Not A Panacea
  • Biological material is rarely left behind by the perpetrator
    • Less than 10% of criminal cases
  • Problems with partial profiles
  • Problems with interpreting mixtures
  • Innocent transfer
  • Doesn’t solve tunnel vision
contact information
Contact Information

Olga Akselrod

Senior Staff Attorney

The Innocence Project


Seth Miller

Executive Director

Innocence Project of Florida