Casework and DNA transfer
The detection of a DNA profile upon a surface cannot be considered proof of contact. DNA is usually carried inside cells. A good idea of the relative size of a cell (and a sperm) compared to other things can be found here.
Transfer mechanisms are generally described as being either direct or indirect. Direct transfer can occur when someone has touched an item. Indirect transfer is when DNA from someone ends up on an item without them having touched it. Such indirect transfer has been demonstrated. Most crucially, it has been demonstrated that a full DNA profile of an individual can be recovered from an item that they have not touched, while the profile of the individual that has touched the item is not recovered.
The amounts of DNA that can be deposited through just one touch is varied. Examination of regularly handled items (such as phones, keyboards, door handles) recovers varying amounts of DNA. Quantities of DNA recovered after secondary transfer also vary and, importantly, fall within the range of quantities observed from direct contact.
Therefore, it is impossible to know from the amount of DNA recovered whether the DNA was deposited by direct contact (either through a single touch or regular contact) or indirect transfer. This makes sense when you consider finding some sauce on a table; did it come from the bottle (direct), or from someone’s hand that had touched the bottle or some food with sauce on it (indirect)?
Research has demonstrated that the quantity of DNA recovered and the quality of DNA profiles obtained are complex issues dependent on many factors.
While direct contact may be the most obvious conclusion, there is insufficient scientific data to establish the most likely mode of transfer in any specific instance. Furthermore, it has also been shown that DNA can be transferred during the forensic examination of items. It may therefore be difficult to rely on the locations of the finding of the DNA to inform on how DNA was deposited.
Although it has become routine in forensic science to analyse an item of clothing to detect DNA that could have come from the wearer, there is actually very little published scientific data on the analysis of so-called ‘wearer DNA’.
Added April 2013:
Dr Georgina Meakin and Professor Jamieson of The Forensic Institute published a peer-reviewed study of the literature on DNA transfer which concluded:
" The experimental data reviewed herein shows that nether the quantity of DNA recovered nor the quality of DNA profile obtained can be used to reliably infer the mode of transfer by which the DNA came to be on the surface of interest."
(DNA transfer: Review and implications for casework. Forensic Science International: Genetics 7 (2013) 434–443)
Added February 2017:
Despite the above considerations, there are those who maintain that it is justifiable to opine on the mechanism of deposition of DNA. Click here for information on that debate.