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DNA Transfer

In this video, Dr Gavin Turbett discusses the concept of DNA transfer.
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DNA transfer is actually a very complicated subject. There are actually many routes or paths by which DNA could be transferred. And of course, the challenge is that in real forensic casework, the truth may not ever be known. In terms of DNA transfer, the first is primary or direct transfer. And this is where biological material is deposited directly from the donor, the source onto another item or surface or person. And some examples of that include touching another person or a surface. Handling an object. Sneezing, coughing or speaking. Bleeding, spitting or ejaculating. Or transfer of DNA onto clothing or jewellery that is being worn. In contrast, indirect or secondary transfer is something that happens after the primary transfer event.
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So there’s been an incident of direct transfer and the DNA is transferred again to a second or even third or fourth surface. And this can get referred to as secondary tertiary, even quaternary transfer. The challenge or the consequence of this is that DNA may be found on an item that that person has never been close to or touched. So, I’ll show some examples. Example one, here blood from person A is deposit onto a surface one and surface one, then comes into contact with surface 2. And the result of that is that blood from person A is found on surface 2, even though they never came into contact with it.
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In this example, person A handles an item, item 1 and person B then handles that same item. So person B will now have some DNA from person A on their hands. And that person B handles a new object item 2. DNA is transferred from person A to item 1 to Person B to item 2. Again, we have DNA from person A being found on item two that they’ve never touched. In this example, we’ve got exhibit 1 that has DNA from person A on it, whereas exhibit 2 has no DNA from person A on it. If the two exhibits were packaged together, then there is a risk that DNA would transfer from exhibit 1 to exhibit 2.
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The result, of course, is that DNA would be found, DNA from person A would be found on exhibit 2. And in this context, we would refer to that secondary transfer from exhibit A. Sorry, exhibit 1 to exhibit 2 as being contamination. And this is why we always recommend that exhibits are packaged separately. DNA may not always be recovered from an item following a direct transfer event. There are a lot of factors that can affect this. These include the amount of DNA that’s transferred, the cellular source. Some sources, such as blood, are considered strong sources of DNA, whereas skin cells are considered weak sources of DNA. The area sampled.
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If the wrong area of the exhibit is sampled, then obviously the DNA won’t be detected. And this is a challenge because often the scientists may be looking for invisible trace evidence. The presence of any other background DNA that was already on the item, whether the barriers are used or worn, such as gloves and whether the DNA that is transferred is lost or degraded over time. And even if there is direct contact, the amount of biological material that is transferred may be too low to detect. And again, there are combination of factors that could come into play here. The nature and type of surface, the strength or duration of the contact itself, individual handler can be highly variable.
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The environmental conditions, any cleaning or washing that may have occurred and the time since the contact took place. It is worth noting, however, that in ideal conditions, if an exhibit is stored cool, dry, protected from sunlight, if it’s not handled, DNA can potentially remain on that exhibit for decades. Is transferred DNA any different to DNA that’s been deposited directly? The reality is no, it isn’t DNA that’s been transferred, whether it’s from primary or secondary transfer to another item has the potential to generate a DNA profile. There may be less DNA following a secondary transfer event, but not always.
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And it’ll be the same DNA profile that is that is obtained regardless of whether that DNA was present as a result of primary transfer or secondary transfer. Likewise, the statistical weighting that can be generated from that DNA profile may also still be very strong, even if the DNA is present as a result of secondary transfer. I’ve adapted an image from the van Oorschot paper, and if we imagine here we have a suspect, an accused person who has deposited DNA onto a surface. So this is primary transfer of DNA onto a surface, and we could potentially test their DNA. So I’ve got parts of some little DNA profiles there to show. We get the DNA profile from the person.
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So that is their reference DNA profile. And then we test the crime stain and we can generate the crime scene, DNA profile, and those two DNA profiles will be the same. And as an outcome of that, we may be able to provide a very strong statistical weighting. And this might be something along the lines of the evidence or the DNA profile is a 100 billion times more likely if it has come from the suspect than if it has come from an unrelated person.
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However, what if we have secondary and perhaps then tertiary DNA transfer. So some of that biological material on surface or exhibit 1 transfers onto someone else’s hand or gloves and then is transferred onto another item. That DNA on item 2 will still give that same DNA profile. And we would likewise be able to assign a very strong statistical weighting of perhaps a 100 billion. Same as we did for the primary transfer event. Now, the likelihood ratio is relating to the question of whose DNA is present, but in this context, that is not the important issue. The important issue is not whose DNA. The question is how did that DNA actually get there?
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And that likelihood ratio of a 100 billion should not be confused with that other question. We don’t know the likelihood ratio of how that would explain how that DNA got there. And what if we had a situation where the primary exhibit wasn’t available, it had been moved or wasn’t detected or recognized, and all we had was the secondary transfer? This would appear or could appear as if we had a primary transfer event from the suspect onto item 2, even though that suspect has never been in direct contact with item 2.
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In the early days of DNA testing, the key question was always in relation to whose DNA has been recovered or whose DNA is on the exhibit, and the likelihood ratios that were provided were seeking to address that question. In some criminal trials today, the identity of the donor of the DNA is accepted, and the more important question will actually be how or when did the DNA get there? And this is particularly relevant as the test methods become more sensitive.

Every touch of an individual leaves a trace amount of DNA. An individual’s DNA can be found on objects they have never touched, purely through secondary transfer. Yet, transferred DNA has the potential to generate a DNA profile of that individual. As DNA profiling techniques become more sensitive in nature, smaller amounts of DNA can generate DNA profiles. It is important to note that interpretation of the DNA profile cannot determine if the DNA was transferred. Statistical analysis may be strong even for profiles that were generated from transferred DNA.

Transfer of DNA is affected by multiple factors:

  • Nature of surface: Higher yields of DNA are more likely to be retained by rough, abrasive surfaces, as opposed to smooth, glossy surfaces. Higher yields of DNA are also more likely to be retained by porous surfaces, as opposed to non-porous surfaces.
  • Strength of contact / Duration of contact: Low DNA yields are more likely when the amount of handling and the friction applied is minimal, while vigorous prolonged contact increases the chance of depositing cellular material. High pressure contact is more likely to result in the transfer of cellular material compared to a contact event with minimal pressure applied.
  • Individual handler: Some people shed more cellular material than others in the same duration of time. Additionally, skin conditions like psoriasis, eczema and dermatitis can increase the amount an individual sheds. The level of personal hygiene of an individual can also impact transfer. It is expected that less cellular material will be present on the hands and available for transfer following hand-washing, as opposed to a hand that had not been washed for an extended period of time.
  • Environmental conditions: Harsh conditions like extreme heat, moisture, sunlight and microbial activity can degrade the DNA and prevent further analysis.
  • Cleaning or washing: The process of cleaning, washing or exposure to cleaning chemicals can remove or break down the DNA so that it cannot be analysed or interpreted.
  • Time between deposition and sampling: Lower yields of DNA are more likely when an item is not sampled immediately. Generally speaking, as the time between deposition and sampling increases, the amount of DNA present decreases due to transfer following contact with another item/person; degradation due to environmental conditions; removal due to washing/cleaning.

Due to the multiple factors that affect DNA transfer and due to DNA techniques that can identify small amounts of DNA, it is practically impossible at this stage to scientifically determine how DNA was deposited onto a surface. Although research is being conducted on determining time since deposition of biological material, it is still at a nascent stage and not yet validated for forensic purposes.

An interesting study with reference to DNA transfer and its impact on casework is the example of Lukis Anderson from California (2012). At the time of the incident, Mr. Anderson was 26 years old with priors for petty offences and history of alcoholism. In a case involving robbery which led to the death of the house owner, Mr Anderson’s DNA was discovered underneath the victim’s fingernails. He was arrested and charged for murder, but he did not have any recollection of the event. As in other similar cases, his criminal record along with the circumstances surrounding the case, and the clinching DNA evidence would have been sufficient for a conviction. However, in this case, the public defender was able to find an alibi for Mr. Anderson. The time at which the offence was committed, Mr. Anderson was admitted in the hospital, as he had collapsed after excessive drinking. By tracking the events of the day, the police investigator incharge, discovered that paramedics who had responded to the crime scene had also treated Mr. Anderson and got him admitted to the hospital, just three hours prior to attending the crime scene. They were able to connect the presence of Mr. Anderson’s DNA in the crime scene to the paramedics who caused a tertiary transfer between Mr. Anderson and the victim.

It is important to note that during casework, it is difficult to ascertain if the DNA profile generated is from primary, secondary or tertiary transfer. Therefore, lawyers must remember that transfer is always a possibility and the opinion of the expert is limited to identifying the source of the DNA. DNA examiners cannot offer any clarity on the method or the order of deposition of DNA.

In this video, Dr Turbett will provide you with further guidance on the issue of DNA transfer.

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Decoding Forensics for Legal Professionals

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