Mechanisms of DNA Transfer
Written by Jane Moira Taupin   

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DNA may be deposited directly on an item which is called direct or primary transfer. DNA may also be deposited on an item indirectly through an intermediary item/s (termed secondary, tertiary or higher level transfer), where there has been no physical contact between the original depositor and the final surface on which the DNA profile was located. Direct, or primary, transfer includes contact but also includes activities within the vicinity of an item such as speaking, coughing and sneezing (Meakin and Jamieson, 2013).

Secondary transfer occurs when a material deposited on an item or person is then transferred to another item or person or onto a different place on the same item/person. There has been no physical contact between the original depositor and the final surface on which the material is located. Any biological substance such as blood, semen, hair, saliva and urine can be transferred like this. Different levels of transfer are depicted in Figure 3.1.

The first study on trace DNA transfer showed that DNA can be recovered from objects touched by hands (van Oorschot and Jones, 1997). The finding regarding touched objects has been much discussed due to its investigative potential. The other findings in this study are just as valuable. Volunteers handling tubes had their hands swabbed, and DNA profiles were observed that matched the previous holders of the tube – the volunteers had not contacted each other. Tubes held for a short time by a second or third person usually provided the DNA profile of the last holder but also provided the DNA of previous holders. The study showed that

  • DNA may be transferred from hand to object (direct transfer) and then from object to hand (secondary transfer)
  • There may be no physical contact between the original depositor and the final surface on which the DNA profile was located (secondary transfer)
  • DNA yields from tubes held for varying lengths of time (5 and 30 seconds; 3 and 10 minutes) did not vary significantly indicating substantial transfer during initial contact
  • Hands swabbed before and after a 1 minute handshake revealed the transfer of DNA from one individual to another in one of the four hands tested – thus DNA was not always transferred
  • Genetic profiles from objects handled by several people or from minute blood stains on touched objects may be difficult to interpret
  • There is a need for caution when handling exhibits and interpreting results
  • Figure 3.2 shows pathways of hand transfer of DNA. A biological substance that has been transferred multiple times, if detectable, may appear as components of complex DNA profiles. This is because the vectors (such as hands or implements) aiding the transfer, and/or the substrate from which it is ultimately collected, may also bear DNA (Goray et al., 2010).

Sometimes the vector may not bear DNA and this could complicate interpretation even further. A study (Fonnelop et al., 2015) found that there could be tertiary transfer when there was no indication of a previous transfer (either direct or indirect.

There is still limited knowledge concerning conditions that may influence secondary or higher transfer. One scientist (Champod, 2013) recently reiterated that there is a need for forensic scientists to highlight how little is known about DNA transfer mechanisms. Several factors may influence secondary or higher transfer of DNA. These include the type of biological substance deposited, the nature of the primary and secondary substrate, the moisture content of the deposit and the type of contact between the surfaces. These factors are those typically considered in the transfer of trace material in general.

FIGURE 3.1—Levels of transfer. Note: Items may transfer material back to the previous items.

FIGURE 3.2—Pathways of hand transfer. Diagram of handing a plastic tube­—consecutive. The hand of person 1 touches the tube and then the hand of person 2 touches the tube. The tube may contain DNA from person 1 plus DNA from person 2. The hand of person 2 may contain DNA from the plastic tube and thus DNA from the hand of person 1.

 
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