Primary DNA transfer is defined as the passage of DNA to objects and people. Secondary DNA transfer is an extension of this process, where DNA is transferred to an object or person through an intermediate. This kind of transfer can lead to individuals being falsely linked to a crime scene and can also reduce the likelihood of prosecuting the real criminal due to the introduction of foreign DNA to a forensic sample.
TheaDesign | Shutterstock
Considerations for secondary DNA transfer
Advancements in DNA profiling have been a double-edged sword for forensics. DNA profiling enables scientists to amplify small traces of DNA with high sensitivity in order to link a person to a crime. However, the drawback is that amplification of such small DNA amounts may not correlate with activity level, meaning DNA can be from someone unconnected to the crime but brought there via secondary DNA transfer.
Understanding what influences DNA transfer, how much genetic material a person of interest is likely to have deposited, and the time since the transfer event is important. This can be complicated by individual variation in depositing or retaining their DNA. Furthermore, while it may seem intuitive to predict that objects with rougher surfaces are more susceptible to secondary DNA transfer, there is limited evidence for this.
Forensic scientists largely struggle to distinguish primary from secondary DNA transfer. Some research has used a Bayesian network model to account for features associated with DNA transfer, persistence, and recovery, thereby permitting forensic analysists to account for the high variability in all three factors.
Understanding secondary DNA transfer
The idea of secondary DNA transfer has been around since the late 1990s, but results were highly inconsistent. This was believed to be due to the highly individualistic and variable capacity to shed DNA which is also variable.
Since the early 2000s, technological advances have improved and yielded more data regarding DNA transfer. Increased advances mean that less DNA is needed to generate full profiles, which can implicate secondary DNA transfer present in minute amounts. There is some evidence that DNA spread does not stop at secondary transfer, with some studies finding tertiary and quaternary DNA transfer.
The most recent DNA testing kits are sensible to secondary transfer. One study indicated that 85% of tests had some degree of secondary DNA transfer. The same study found that in one-fifth of those cases, the DNA by secondary transfer was identified as the only DNA present or as the main contributor of the DNA present.
Effect on convictions
High sensitivity DNA kits have only been made available over the last decade. Such kits are now being used to reinvestigate unsolved cases in which minute DNA traces were discovered.
Secondary DNA transfer has also been successfully used to overturn wrongful convictions. After a murder in California, USA, a man was charged with murder and spent months in prison before his alibi proved he was innocent. His DNA had been transferred to the crime scene via first responders. Many professionals agree that increased knowledge of secondary DNA transfer will be highly important in future cases.