Challenging Samples
- Created by: AAntonianannetti
- Created on: 13-05-19 17:38
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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)
- Majority of studies showed that it was not always possible to leave behind DNA
- Studies done in order to show if this is true and all touch DNA is detectable
- Every contact leaves a trace
- 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
- Look at 17 alleles
- Not used now as DNA 17 has increased sensitivity by useful to understand issues
- Alleles not reported unless they are seen in at least two runs
- 2 runs serves as a safeguard against allelic drop ins
- Contamination
- 2 runs serves as a safeguard against allelic drop ins
- DNA profiling performed at or beneath the stochastic threshold
- 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
- Occurs when the evidence contains mixtures of DNA from several people
- 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
- Important as a common argument can be that DNA is explained by an earlier presence at the scene
- Need for other circumstantial evidence
- Prosecution
- Touch DNA
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