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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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Olga Akselrod Senior Staff Attorney The Innocence Project

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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?

  • 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 DNA?

  • Nucleas = 46 Chromosomes

    • 23 from egg (mother)

    • 23 from sperm (father)

  • Many Mitochondria

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 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

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

  • Extraction

  • Quantification

  • Amplification

  • Electrophoresis

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

  • 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

  • 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 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 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 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

  • 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 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 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 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 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 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 . . .

  • 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 . . .

  • 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 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

    • 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

    Olga Akselrod

    Senior Staff Attorney

    The Innocence Project


    Seth Miller

    Executive Director

    Innocence Project of Florida


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