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dna analysis & interpretation > Contamination > Flashcards

Contamination Flashcards

1
Q

What is meant by the term RFU?

A

Relative Fluorescent Units
* on the y axis of a EPG
* indicates the intensity

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2
Q

What is sodium hypochlorite used for in DNA analysis?

A
  • decomtamination and sterilisation purposes
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3
Q

The allele count at a particular locus represents the number of repeating what?

A

base pairs

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4
Q

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?

A

Minisatellites

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5
Q

How is the new DNA17 multiplex making contamination easier?

A
  • very sensitive
  • capable of detecting ever smaller amounts of DNA
  • the improved sensitivity means contaminated DNA is more lkely to be detected
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6
Q

What is the definition of contamination?

A

to make something impure by exposure to or addition of a polluting substance

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7
Q

What is adventitous transfer?

A

transfer from an innocent activity before the crime event

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8
Q

How is contamination and advantageous transfer different?

A
  • contaminatin: any time after crime event
  • advantageous transfer: before crime event
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9
Q

What is the issue with contamination if it goes undetected?

A
  • 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
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10
Q

What are the two constituents that contamination could manifest as?

A
  • major
  • minor
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11
Q

What are the two types of contamination that it could be?

A
  • one off - affecting a single tube in a batch of samples
  • blanket - affect the whole batch of samples in a gross or blanket contamination
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12
Q

What are the 3 criteria that need to be fulfilled for contamination?

A
  1. source of DNA available
  2. opportunity for DNA transfer to occur
  3. a mechanism by which the DNA transfer is achieved
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13
Q

What are the first potential sources of contamination?

A
  • police
  • scenes of crime officers
  • pathologists
  • forensic sciences
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14
Q

What are the second potential sources of contamination?

A
  • consumables
  • reagents
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15
Q

Where are the opportunies for contamination?

A
  1. from person to stain
  2. from consumable to stain
  3. from stain to stain
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16
Q

What are the three mechanisms for transfer?

A
  1. direct primary contact
  2. secondary transfer
  3. tertiary
17
Q

How can you prevent contamination transfer?

A
  • training - knowledge of what could happen and where to look for it
  • procurement of consumables from a source supported by quality standards
18
Q

What detection methods help prevent contamination?

A
  • elimination databases for laboratory staff
  • quality checks
19
Q

What is allele drop in?

A

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

20
Q

How could allele drop-ins occur?

A
  • dust particles
  • plastic ware
21
Q

What causes differences in the peak heights of repeated application of the same input to DNA?

A

stochastic effects

22
Q

What is touch DNA?

A
  • trace DNA
  • requires very small samples for analysis
  • skin cells can be transferred from just touching objects
23
Q

What is a case that shows the issue of touch DNA?

A
  • 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
24
Q

The case of Gareth Williams

A
  • person to sample contamination
  • caused a misled investigation and wasted resources
25
Q

Phantom of Heilbronn

A
  • 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

dna analysis & interpretation (17 decks)

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