Contamination

Question 1

1. The statistics associated with full 17 loci matches will be reduced in cases where we encounter incomplete profiles, relatives and what else?
2. What is missing from this sentence? What is the chance that a ———— ———- unrelated person would have the same DNA profile that we are seeing in the evidence sample?
3. In contrast to the reference sample, we might expect to see a mixture in the ——— sample
4. The allele count at a particular locus represents the number of repeating?
5. Where only one peak is seen on the EPG, this is said to be?
6. During the Pitchfork case, Jeffreys used a type of repeating (DNA) 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?

Answer 1

1. The statistics associated with full 17 loci matches will be reduced in cases where we encounter incomplete profiles, relatives and mixtures.
2. What is missing from this sentence? What is the chance that a randomly chosed, unrelated person would have the same DNA profile that we are seeing in the evidence sample?
3. In contrast to the reference sample, we might expect to see a mixture in the evidence sample.
4. The allele count at a particular locus represents the number of repeating base pairs.
5. Where only one peak is seen on the EPG, this is said to be homozygous.
6. During the Pitchfork case, Jeffreys used a type of repeating (DNA) 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.

Question 2

Why is contaminated DNA more likely to be detected

Answer 2

• The new multiplexes make the new DNA 17 multiplexes very sensitive
• Capable of detecting ever smaller amounts of DNA.
• The improved sensitivity of the new multiplexes means contaminated DNA is more likely to be detected.
• Understanding the mechanisms for contamination is an important part of a DNA 17 assessment.

Question 3

Oxford dictionary definion of contamination

Answer 3

To make something impure by exposure or addition of a polluting substance.

Question 4

The forensic process

Contamination

Answer 4

• Contamination is applied to the transfer of foreign DNA to the sample after the crime has been discovered.
• In this respect therefore, the term contamination is applied to the forensic process from scene attendance through to sample collection and sample testing
• Contamination does not relate to transfer from an innocent activity before the crime event.
• This is regarded as adventitious transfer and includes background levels of DNA for example, the DNA on a garment from the person who might have habitually worn it.

Question 5

Contamination

If undetected

Answer 5

If undetected
1. Complicates interpretation
2. Increases the risk that a result which is incorrectly attributed
3. Or given undue significance
4. Potentially diminishing the value and reliability of the forensic evidence
How the contamination event affects the results depends upon the amount of contamination.

Question 6

Consequences of contamination

Answer 6

• Could turn a single source sample into a mixed one
• Turn a sample with no result into a false positive.

Question 7

Depending on the level of contamination

Answer 7

It could be a major or minor constituent.

Question 8

One off versus blanket contamination

Answer 8

• It could be an incident that happens once and affects a single tube in a bacth of samples
• It could affect a whole batch of samples in a blanket contamination.

Question 9

The DNA contamination event

Three criteria

Answer 9

Contamination, like any other DNA transfer event, requires three criteria to be fulfilled.
1. Firstly, there needs to be a source of DNA available to be involved in the transfer event
2. There needs to be an opportunity for DNA transfer to occur
3. There needs to be a mechanism by which the DNA transfer is achieved

Question 10

Who does the first potention source of contamination concern?

Answer 10

Concerns the individuals involved in the different roles of the forensic process such as
* Police
* Scenes of Crime Officers
* Pathologists
* Forensic Scientists etc.

Question 11

What is the second category that can cause contamination?

Answer 11

• Is that of the contaminating DNA originating from any consumables or reagent used in the forensic process and the environment in which the test is carried out.
• It is important to understand where our consumables are produced and the quality standards which are associated with their manufacture.
• Finally, we consider other forensic samples which are being processed concurrently within the laboratory environment.
• In this respect, having a quality managed lab process with appropriately trained staff is an essential requirement.

Question 12

Opportunities for contamination

Answer 12

The opportunities for contamination can therefore be summarised in three different ways:
1. From person to stain
2. From consumable to stain
3. From stain to stain

Question 13

The mechanisms for transfer

Answer 13

The mechanisms can operate by direct primary contact events or higher-level secondary transfer events.

Question 14

Prevention of contamination

What does it require?

Answer 14

• Forensic practitioners can introduce tools to prevent and detect contamination should it occur.
• This requires knowledge of what could happen and where to look for it. In turn, this requires an understanding of how people work; the nature of consumables in use and the environment in which we operate.

Question 15

Reducing opportunities for contamination

Answer 15

• Our processes and tools we use are aimed at reducing the opportunities and disrupting the mechanisms for contamination.
• For example, preventative measures may include the training of staff to a recognised standard and procurement of consumables from a source supported by quality standards.
• Detection methods include the introduction of elimination databases for laboratory staff as well as appropriate quality checks within the forensic process.

Question 16

When can contamination occur?

Answer 16

Contamination can occur at collection, extraction, amplification and injection.

Question 17

What increases the risk of contamination?

Answer 17

Degraded or low templated DNA increases the risk.

Question 18

Drop-in

Answer 18

• A form of profile contamination were one or two alleles are present in the DNA profile that are not inherent to the DNA extract.
• Not reproducible on reamplilification and may be from dust particles from the laboratory environment or plastic ware used within the DNA process.

Question 19

1. The timeline of events from which the crime occurs to which the potential for contamination could be experienced is referred to as?
2. Contamination can change a single source DNA sample into a?
3. An incidence of contamination could be a one-off event or could be an example of
4. Contamination, like any other transfer events requires three criteria to be fulfilled. The first of these is referred to as?
5. We refer to contamination events which occur before the crime is detected as?
6. The opportunities for contamination can be summarised in three different ways. From person to stain, from consumable to stain and what else?

Answer 19

1. The timeline of events from which the crime occurs to which the potential for contamination could be experienced is referred to as the forensic process.
2. Contamination can change a single source DNA sample into a mixture.
3. An incidence of contamination could be a one-off event or could be an example of blanket contamination.
4. Contamination, like any other transfer events requires three criteria to be fulfilled. The first of these is referred to as source.
5. We refer to contamination events which occur before the crime is detected as adventitious transfer.
6. The opportunities for contamination can be summarised in three different ways. From person to stain, from consumable to stain and stain to stain.

Question 20

Touch/trace DNA

Answer 20

• Touch DNA is a type of DNA analysis at a crime scene which only requires 7-8 human skin cells.
• Requires very small samples for analysis
• Skin cells can be transferred from casual touching of objects.
• This has been recently criticized for false positives due to contamination.

Question 21

Jon Benet Ramsey

Touch DNA

Answer 21

• In 2008, the parents of murdered child, Jon Benet Ramsey, were cleared after an unknown male’s DNA was found in her underwear.
• The sample was so small it might have been from the person at the factory packaging the underwear.

Question 22

Gareth Williams

Answer 22

• Contamination is not just a theoretical concept and there have been several high- profile examples which serve to illustrate the impact these events can have on the criminal justice system.
• This recent person to sample example of contamination involved the transfer of DNA from a Scenes of Crime Officer to an exhibit. Subsequent testing produced a DNA profile which caused misleading investigation and wasted resource.

Question 23

Phantol of Heilbronn

Answer 23

• From 1993 to 2009, detectives matched the DNA of a serial killer in over 40 crimes ranging across Austria, France and Germany . The crimes included burglary and six murders.
• In 2007, Police Officer Michele Kiesewetter was murdered in Germany and the DNA matched to the unknown serial killer. The DNA sample found at the crime scene came from an Eastern European woman.
• An investigation revealed that the cotton swabs used to collect DNA were not certified for human DNA collection, & sterile
  All of the cotton swabs came from the same factory.
• The Phantom serial killer did not exist; the worker’s DNA was on the cotton swab before police received them.

Facts

Question 24

Phanrom of Heilbronn

Example of consumable sample contamination

Answer 24

• Involves the transfer of DNA from consumables, specifically cotton swabs that were used in the collection of samples.
• Procurement of the swabs by many laboratories from a common manufacturer gave results which provided false information regarding the case linkage in what were actually unconnected cases.
• The source of the DNA was eventually traced to a female employee involved with the manufacture of the swabs.

Question 25

Manchester Rape | Environmental contamination

Answer 25

* A recent example of a sample to sample contamination event which led to the wrongful arrest of an individual. * An investigation into the incident, showed that the failure in laboratory practice which involved the re-use of sampling equipment resulting in DNA profiles being given inappropriate relevance in the context of the case.

Question 26

1. What type of contamination might we encounter by the poor use of PPE? 2. What mechanisms do forensic laboratories use to prevent contamination events occurring? 3. What type of samples increase the risk of contamination? 4. We may see differences in peak height between repeated applications of the same input due to these effects? 5. A form of profile contamination where one or two alleles are present in the DNA profile that are not inherent in the DNA extract? 6. What procedures do forensic laboratories put in place to detect contamination?

Answer 26

1. We would get person to stain contamination might we encounter by the poor use of PPE? 2. What mechanisms do forensic laboratories use to prevent contamination events occurring? Training and procurement. 3. What type of samples increase the risk of contamination? Degraded or low template 4. We may see differences in peak height between repeated applications of the same input due to these effects? Stochastic 5. A form of profile contamination where one or two alleles are present in the DNA profile that are not inherent in the DNA extract? Drop in 6. What procedures do forensic laboratories put in place to detect contamination?Elimination databases and quality checks