Topic 6 - Contamination Flashcards
what is the forensic process
the timeline from when a crime was committed and when a match is suggested
what is contamination here
the transfer of foreign DNA to a sample after the crime has been discovered
when can contamination of samples occur
any time from scene attendance to sample collection and sample testing
what does contamination not relate to
the transfer due to activities before the crime event
what are included in adventitious transfers
background DNA levels before the crime event
- on a garment for example of the person who wore it
if undetected what can contamination cause complication in
data interpretation (DNA profile) diminishing the value of the evidence
dependent on the amount of contamination that has occurred
what change can contamination to
a) a single source profile
b) a sample that would have given no result
a) makes it into a mixture so it is harder to interpret
b) a sample giving a false positive result
what two types of incident can contamination be classed as
a one off event = affecting only one sample tube
a blanket = affecting a whole batch of sample
what three things are needed for contamination to occur
(the same for any type of DNA transfer)
- source of the contamination
- opportunity for transfer
- mechanism for transfer
depending on the degree of the contamination what two things can this represent in a DNA profile mixture
the major or the minor contributor
where does the first potential source of contamination lie in forensic investigation
police, crime scene officer, pathologists or scientists (in this order)
i.e the people handling the sample first
where is the second source of contamination of sample
why should these not cause contamination
in the consumable or reagents used in the forensic process or other samples around or packaging
these should be of a certain quality standard so shouldn’t cause contamination and the staff should be appropriately trained
what are the three opportunities for contamination in summary
person to stain
consumable to stain
stain to stain
what is the mechanism of transfer for contamination
give three example of how this transfer can happen
via direct contact or secondary transfer
-poor use of PPE
- not cleaning equipment or workbenches
- cross contamination of sample
what does the prevention of contamination require the knowledge of (3)
how people work
the nature of the consumables being used
then environment being operated in
what can be done to prevent contamination (2)
better training
use of consumable that are quality standard
what can be done to detect contamination (2)
using elimination databases
quality checks
what can be looked at to see if contamination has occurred
DNA profiles compared to positive and negative controls
what type of DNA sample are more at risk of being affected by contamination
low template or degraded samples
what is drop-in
how do we deal with suspected drop in
a form of contamination where 1 or 2 alleles are present that are no inherent to the sample
repeat the profiling process if the allele is not observed then exclude it from calculations
what is touch DNA
why has this been recently criticised
type of DNA analysis that only requires 7-8 human skin cells - allows analysis of very small samples
looking at the DNA transfer simply by touching an object
criticised by the observation of false positives due to contamination
