Challenging Samples

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  • Challenging Samples
    • Touch DNA
      • The deposition of just a few microscopic calls from a perpetrator onto a victim or an item at the ctime scene which can be enough to  allow for the development of a DNA profile
      • DNA testing is extremely sensitive and informative
      • Testing of touch DNA cannot tell us how DNA came to be in a specific location
      • Factors that influence touch DNA
        • Time
        • Hand washing/personal habits
        • Type of contact
        • Substrate
        • Perspiration
      • Locards Exchange Principle
        • Every contact leaves a trace
          • Studies done in order to show if this is true and all touch DNA is detectable
            • Majority of studies showed that it was not always possible to leave behind DNA
              • 51-70% of individuals failed to leave behind their DNA after holding a sterile tube of 10 seconds (Phipps and Petricevic, 2006)
      • How likely?
        • Person to person to object
        • Person to object to object
        • Person to object to person
        • Can be related to the issue of: Can the DNA of my client be present on the evidence item by means other than direct contact
    • Low Number Copy (LNC)
      • DNA profiling performed at or beneath the stochastic threshold
        • Not used now as DNA 17 has increased sensitivity by useful to understand issues
          • Look at 17 alleles
            • 16 normal alleles and 1 from the sex chromosome
      • Alleles not reported unless they are seen in at least two runs
        • 2 runs serves as a safeguard against allelic drop ins
          • Contamination
    • Mixtures
      • Occurs when the evidence contains mixtures of DNA from several people
        • A profile can be generated from a small sample
      • Some mixtures are difficult to interpret
      • Example
        • Meredith Kercher (2007)
      • Analysing a DNA mixture
        • DNA must be amplified in order to analyse allele variation in establishing a DNA profile
        • Mixtures of DNA will create more smaller peaks when analysed
          • This creates uncertainty and suggests that there is more than one person
          • Alleles from all contributors show up on the same chart and this can make it difficult to identify individual DNA profiles
            • causes uncertainty in understanding who the suspect is as just because an individuals alleles appear in the mixture does not mean that they contributed to it
    • Can somebody's DNA  be found at a crime scene without the person having been present?
      • Yes
    • DNA in court
      • Prosecution
        • Two samples have the sample source
      • Defense
        • An error has occurred / samples match coincidentally
      • Persistence of touch DNA cannot be determined
        • Important as a common argument can be that DNA is explained by an earlier presence at the scene
          • Other factors of persistence should be considered
      • Need for other circumstantial evidence


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