Contamination Flashcards
What is meant by the term RFU?
Relative Fluorescent Units
* on the y axis of a EPG
* indicates the intensity
What is sodium hypochlorite used for in DNA analysis?
- decomtamination and sterilisation purposes
The allele count at a particular locus represents the number of repeating what?
base pairs
During the Pitchfork case, Jefferys used a type of repeating sequences, different to the ones used today. The repeated segments, in this case dozens or even 100 bases long can be observed. These are referred to as?
Minisatellites
How is the new DNA17 multiplex making contamination easier?
- very sensitive
- capable of detecting ever smaller amounts of DNA
- the improved sensitivity means contaminated DNA is more lkely to be detected
What is the definition of contamination?
to make something impure by exposure to or addition of a polluting substance
What is adventitous transfer?
transfer from an innocent activity before the crime event
How is contamination and advantageous transfer different?
- contaminatin: any time after crime event
- advantageous transfer: before crime event
What is the issue with contamination if it goes undetected?
- complicates interpretation
- result could be incorrectly attributed
- result could be given undue significance
- diminish the value and reliability of the evidence
- could change a single source into a mixture
- could give a false positive result
What are the two constituents that contamination could manifest as?
- major
- minor
What are the two types of contamination that it could be?
- one off - affecting a single tube in a batch of samples
- blanket - affect the whole batch of samples in a gross or blanket contamination
What are the 3 criteria that need to be fulfilled for contamination?
- source of DNA available
- opportunity for DNA transfer to occur
- a mechanism by which the DNA transfer is achieved
What are the first potential sources of contamination?
- police
- scenes of crime officers
- pathologists
- forensic sciences
What are the second potential sources of contamination?
- consumables
- reagents
Where are the opportunies for contamination?
- from person to stain
- from consumable to stain
- from stain to stain
What are the three mechanisms for transfer?
- direct primary contact
- secondary transfer
- tertiary
How can you prevent contamination transfer?
- training - knowledge of what could happen and where to look for it
- procurement of consumables from a source supported by quality standards
What detection methods help prevent contamination?
- elimination databases for laboratory staff
- quality checks
What is allele drop in?
a form of profile contamination where one or two alleles are pesent in the DNA profiles that are not inherent to the DNA extract
* not reproducible on reamplification
How could allele drop-ins occur?
- dust particles
- plastic ware
What causes differences in the peak heights of repeated application of the same input to DNA?
stochastic effects
What is touch DNA?
- trace DNA
- requires very small samples for analysis
- skin cells can be transferred from just touching objects
What is a case that shows the issue of touch DNA?
- the parents of a mudered child (Jon Ramsey) were cleared after an unknown males DNA was found in her underwear
- but this sample was so small it could have been from the person who packed the underwear in a factory
The case of Gareth Williams
- person to sample contamination
- caused a misled investigation and wasted resources
Phantom of Heilbronn
- the workers DNA was on the cotton swab before police received them
- swabs were not certified for human DNA collection and sterile
- DNA matched to over 40 crimes
