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Transfer Theory in Forensic DNA Analysis

A review of primary, secondary and tertiary transfer



Suzanna Ryan | Monday, January 12, 2009

Imagine the following scenario as you are exiting your local bank, a man you do not recognize stops you with a hearty "hello" and a handshake. Soon, the man realizes he has mistaken you for someone else and continues into the bank. Later that day you hear that there was a robbery at the very bank you visited. A few days later, the police are knocking on your door ready to arrest you for the robbery. The police even have your DNA at the crime scene. In fact, it was found on the pen used to write the robbery note. You are certain you never touched that pen so how could this be possible? Perhaps you didn't. But, that man at the entrance to the bank that shook your hand? Well, he did touch the pen. And the DNA you transferred to his hand via the handshake, he then transferred to the pen. Is this science fiction? Or, is it a possibility in the modern forensic DNA lab?

Forensic DNA analysts and researchers have learned in recent years that it is possible to obtain DNA results from items that have merely been touched or handled by an individual. Now, scientists, police, and attorneys alike want to know if it is possible for DNA to be transferred one more step, in what has been termed secondary transfer.

Primary transfer of DNA

Primary transfer is described as the transfer of DNA from an individual to an item. For example, when a person touches the handle of a knife, we know that, depending on a number of variables, it is possible for DNA-bearing cells to slough off the person's hand and adhere to the surface of the handle. This primary transfer is the basis of many police agencies decisions to collect and test "touch DNA" evidentiary items. Just about any item a suspect has handled can potentially result in a DNA profile. This includes items like guns, knives, steering wheels, door handles, window frames, writing utensils, telephones, clothing items and so on. Through studies by Ladd, et al and others, many in the forensic community believe that some individuals just naturally shed more cells when handling an item ("good" shedders) than others ("poor" shedders). Other factors including the substrate being handled (rougher items collect more DNA), the time since the individual last washed their hands, how nervous the person is (nervousness can lead to increased sweating), and how often they touch their mouth, eyes, hair, face or other body parts (thus gathering DNA on their hands) play a heavy role in whether a DNA profile will be obtained through simply touching an item.

Secondary transfer of DNA

In secondary transfer, there is no direct contact between a person and an object. It is instead transferred through an intermediary. This could be another person or another object. An example expanding upon the same knife example as above is as follows: Person A shakes Person B's hand. Person B touches the handle of a knife. Secondary transfer theory would allow that Person A's DNA could be transferred first to Person B's hand and second to the knife handle. This would mean that even though Person A never actually touched the knife handle, his DNA could be present on it.

Although a review of the literature dealing with transfer of DNA shows contradictory results, at least some of the studies show that in laboratory conditions secondary transfer is indeed possible. One study performed by Lowe, et al, was designed to highlight a "worst case" scenario and involved two individuals. The first was determined to be a poor shedder and the second a good shedder. These two shook hands for one minute. The poor shedder had washed their hands immediately prior to the experiment whereas the good shedder had not. After shaking hands the poor shedder held a sterile plastic tube for 10 seconds. The tube was then swabbed and tested for the presence of DNA. This experiment was performed on two sets of good shedder/poor shedder pairs. Surprisingly, in one of the pairs, only the good shedder's DNA was obtained from the plastic tube, with no evidence of a mixture including the poor shedder!

Other studies (Phipps and Petricevic) have shown little to no secondary transfer in various experimental conditions. They also show that it is difficult to classify individuals as either "good' or "poor" shedders as the same individuals tested on various days showed differing primary transfer (from hands to sterile plastic tube) results. At some times, DNA was transferred and at other times, the same individuals showed no transfer of DNA.

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