Is secondary DNA transfer a possibility in this case?
Another common question in regards to touch DNA analysis is, “Is it possible that my client’s DNA arrived on the item of evidence via secondary transfer?”. In other words, could someone’s DNA be found on an item even though they never actually touched that item? Based upon several recent studies, the answer is, yes, this is indeed possible.
Secondary transfer can be from:
person to person to object – I shake your hand and my DNA is transferred via primary transfer to your hands. You then pick up a knife and transfer my DNA to the knife (secondary transfer)
person to object to person – I pick up and swing a wooden bat and transfer my DNA to the handle (primary transfer), you pick up the same bat and a swab of your hands reveals my DNA (secondary transfer)
person to object to object – I use a bath towel (primary transfer of my DNA to the towel) that is then wrapped around a gun and my DNA is found on the gun (secondary transfer from towel to gun).
One of the initial studies on secondary DNA transfer- after the original short correspondence describing examples of secondary transfer published by van Oorschot and Jones in Nature in 1997 – was a study by Ladd et al (1999) who appear to have sought to dispel the possible interference of secondary DNA on an item of evidence. They conclude their paper by stating that “Secondary transfer was not observed under our experimental conditions”. However, a closer look reveals that low-level secondary transfer was indeed observed. The authors note that, “on occasion, minor peaks (below 75 RFU) from the second individual were observed”. This is secondary transfer. In addition, as mentioned above, the increased sensitivity common with today’s methods and techniques would likely have yielded an even greater appearance of secondary DNA transfer in the Ladd study. More recent studies indicate that secondary transfer of DNA can and does occur under varying circumstances.
Daly et al (2010) studied DNA transfer of touch DNA onto three substrates: wood, glass, and fabric (100 samples each). Of the 300 samples, they found that mixed DNA profiles (indicating a person to person to object secondary transfer) were obtained in ~10% of the samples. Interestingly, they note two instances in which a male profile or a major male/minor female profile was obtained from samples held by female individuals. These results echo the results of a study by Lowe et al (2002) wherein an individual held hands with a second individual and then handled a pre-sterilized plastic tube and transferred only the second individual’s DNA to the tube, none of their own.
A detailed study of secondary DNA transfer of skin cells was performed by Goray et al (2010). These researchers found that freshly transferred skin cells transferred to a secondary surface more easily than dried transfers and that non-porous primary substrates (such as plastic and glass) generate increased transfer rates whereas porous secondary substrates (cotton was used in the experiment), “facilitate significantly greater transfer compared to non-porous ones”. In addition, it was found that transfer rates approximately double when pressure is involved in the transfer, as opposed to a passive contact, and that transfer rates increase even further when friction is used. The authors used their findings to estimate the amount of primary DNA deposit that would be necessary to allow for the secondary transfer of 1 nanogram of DNA. The authors state:
The amount of original deposit will vary significantly depending on substrate, manner of contact and, in some instances, the freshness of deposit. If, for example, a fresh sample was deposited on cotton (soft and porous) and then subjected to pressure contact (with no friction) with a secondary substrate that is hard and non-porous, like plastic, a minimum deposit of 385ng of DNA is needed for 1ng to have been transferred, collected, and extracted….Whereas, if the type of primary and secondary substrate was reversed (primary was hard, non-porous and secondary soft, porous), and in addition the sample was dried prior to contact that involved friction, then only 2ng of DNA needs to be deposited for 1ng to be retrieved.
An example of how secondary transfer could conceivably affect the interpretation of a case was illustrated by Sarah Jones and Kirsty Scott of the SPSA Forensic Services (Aberdeen) during the Body Fluids Conference jointly hosted by the Forensic Science Society & the Centre for Forensic Investigation, University of Teesside and reported in Science and Justice 50 (2010). Jones and Scott performed experiments to determine if non-intimate contact could result in the transfer of DNA to a male volunteer’s underwear and penis. Of three scenarios reported, one resulted in the transfer of the female volunteers’ DNA to both the underwear (33% of the samples) and penis (67% of the samples) of the male volunteers even though no direct contact from the female to the male had occurred. The scenario involved 1 minute of face-touching, 3 minutes of handholding and immediate urination by the male. However, when a 15 minute period was introduced between the non-intimate contact and urination, no female DNA was detected on either the underwear or penis of the male volunteers.
It is also possible for DNA to transfer from person to object to object. In fact, a recent study by Goray et al (2011) discusses the DNA transfer that can occur within forensic exhibit packaging. Their study illustrated some startling results. The authors prepared numerous “mock” evidence samples and packaged and handled them in a manner to mimic the movement of evidence packaged at the crime scene and transported to the laboratory. The results showed the transfer of skin cells from a swab to the interior of the packaging (a plastic tube). In addition, saliva from cigarette butts was found to transfer to other cigarette butts packaged in a manila envelope, and blood stains were shown to transfer to other areas on the same item and to other items packaged in the same container as well as onto the interior of the packaging itself. In addition, bloodied knives showed the transfer of DNA containing material from the tip of the knife to to other areas of the knife including the handle.