Transfer Theory in Forensic DNA Analysis


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.

Tertiary transfer of DNA?

Although secondary transfer of DNA has been observed in laboratory experiments, some want to know if it is possible to add one additional transfer and demonstrate tertiary transfer. One high profile case (MA v. Greineder) involved a prominent physician and adjunct Harvard professor who was accused of killing his wife. A DNA profile consistent with a mixture of the doctor and his wife had been found on gloves and a knife found near the crime scene. Greineder hired a non-accredited private DNA lab to test his hypothesis of tertiary transfer. He proposed that his DNA had ended up on the glove due to transfer. He and his wife had shared a towel the morning of the murder. He argued that his DNA could have been first transferred to the towel and then later to his wife's face (secondary transfer). His wife was attacked by a person wearing gloves at which point his DNA could have been transferred from his wife's face to the gloves (tertiary transfer). The private lab contracted by Dr. Greineder's defense team performed an experiment and presented testimony that indicated that this tertiary transfer could indeed have occurred as the doctor described it. However, the results were never published in a peer-reviewed journal, nor have they been replicated as far as this author is aware.

Secondary transfer at the scene and the lab

Unlike tertiary transfer, the very real possibility of secondary transfer does pose the potential for problems both in the lab and at the crime scene. Consider these examples if you will:

  • A father is accused of molesting his young daughter. The rape kit is negative, but the daughter's underwear, collected from the laundry basket, shows the presence of a very few sperm heads. Could it be possible that secondary transfer has occurred from bedsheets or other laundry to the daughter's underwear during the time the items were in the laundry basket together? Surprisingly, one study (Kafarowski, et al) indicates it is even possible to locate sperm heads after a semen-stained item has been washed! In this same study, a semen-stained item was washed with numerous new, unworn pairs of underwear. Sperm heads were located on the majority of the underwear, thus indicating both the transfer of DNA bearing-cells in the washer and the persistence of sperm heads.
  • At the scene of the crime, numerous beer cans are collected. All the beer cans are packaged together, instead of separately. Later, testing at the lab indicates a mixture of DNA on various cans. The question could be raised as to whether the DNA could have transferred from one can to another during the time they were packaged together. Although a definitive answer does not exist as to whether this could have occurred, the bottom line is that it is possible.
  • At the laboratory, the analysts are careful to wear gloves when handling evidence items. Great care is generally taken to clean scissors, tweezers, and other utensils between testing items. But what about items that are not cleaned as regularly? For example, is it possible to transfer DNA from an item of evidence to a ruler, and then when the next item of evidence is examined and photographed, could the DNA transfer again from the ruler to this item? A 2006 study by Poy and van Oorschot showed an example of secondary transfer when a mixed DNA profile was found on a swab taken from an examination magnifying lamp. This profile was searched in the lab's database and a match was found with a case that had been worked on the bench-top with the magnifying lamp. It was determined that DNA was transferred from the item being examined to the analyst's gloves and then onto the top of the magnifying lamp.

Obviously, the inadvertent transfer of DNA is an area that should be further studied. Since so many of the available journal articles present conflicting information, more work is needed to see how likely it is to both transfer and detect DNA in a secondary or even a tertiary fashion, especially considering the sensitivity of modern forensic DNA analysis.


  1. Kafarowski E, Lyon AM, Sloan MM. The retention and transfer of spermatozoa in clothing by machine washing. Can Soc. Forens. Sci. J. Vol. 29. No1 (1996) pp. 7-11.
  2. Ladd, C, Adamowicz MS, Bourke MT, Scherczinger CA, Lee HC. A systematic analysis of secondary DNA transfer. J Forensic Sci 1999;44(6) 1270-1272.
  3. Lowe A, Murray C, Whitaker J, Tully G, Gill P. The propensity of individuals to deposit DNA and secondary transfer of low level DNA from individuals to inert surfaces. Forensic Sci. Int. 129 (2002) 25-34.
  4. Phipps M, Petricevic S. The tendency of individuals to transfer DNA to handled items. Forensic Sci. Int. 168 (2007) 162-168.
  5. Thompson, WC, Ford, Doom, TE, Raymer, ML, Krane, DE. Evaluating Forensic DNA Evidence, Part 2. Champion Magazine, May 2003:24-27
  6. Walton L, Jagho R, Mountain H, Jackson A. The secondary transfer of DNA: the influence of shedder status on the dominant donor in a mixed profile. Poster presented at the 'Advances in Forensic DNA Analysis Conference', University of Teesside, September 2006.


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