DNA Transfer

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.