Questions frequently arise about touch/transfer/trace DNA. Here I post some articles on the topic as a resource for interested individuals. Research continues and there are constantly new studies being performed. There are so many variables, however that in addition to consulting the literature; one has to consider some of the basics. I’ll offer a quick rundown on some key points to complement the articles I am providing.
DNA is found within nucleated cells in the human body. The human body is composed of trillions of cells. Body fluids such as semen, blood, and saliva contain large amounts of nucleated cells and therefore are rich in DNA. In semen, a vast majority of the DNA is found in the sperm cells. In blood, the DNA is found within the white blood cells (red blood cells do not have nuclei). Other body fluids/sources do not necessarily have characteristic proteins or cells that can be easily associated exclusively with the body fluid. For example, epithelial cells are common throughout the body as they line cavities such as the mouth, rectum, vagina as well as many other places.
Urine can contain nucleated cells that are accumulated as urine exits the body. Perspiration also contains cellular material that is sloughed off and carried out of the body. There are cells sloughed off from the eyes which can transfer to our hands when we wipe our eyes. Cells on the skin itself may contain DNA.
From Locard’s Exchange Principle (a forensic science fundamental found in any criminalistics textbook) we know that when an individual comes into contact with another person or place (e.g. crime scene), there is a cross-transfer of evidence. In plain terms, the individual leaves something at the scene and takes something from the scene. This transfer might be obvious, such as an individual leaving fingerprints at a crime scene and leaving with the victim’s blood on their hand, or it may be more subtle. The transfer might involve a single hair, particles of dirt, or fibers. This transfer might be completely invisible and cellular in nature. As DNA testing has increased in sensitivity, more of these cellular transfers are detectable.
Given that the human body has trillions of cells, there are many mechanisms for DNA to be on a person’s hands (perspiration, skin cells, rubbing the face or eyes, etc.), and the sensitivity of today’s DNA testing, we are forced to consider touch DNA and transfer when interpreting crime scene DNA evidence.
As to how likely it is that an innocent touch of a person or object will result in a DNA transfer, the variables are numerous. In fact, some individuals just tend to ‘shed’ their DNA more than others. This can be a biological variable or behavioral (think more sweating, more face rubbing, etc.). All things being equal….the more time a person is in contact with something, the more DNA they will likely deposit. However, consider an example of two people, each wearing a different ski mask for the same period of time. Will they leave the same amount of DNA? Of course not.
To demonstrate how easily DNA can transfer via innocent means, one need look no further than the forensic DNA laboratory. Extraordinary measures are used to ensure that among other things, the analyst’s DNA does not contaminate evidence they are working on. I can tell you that it happens, and more often than you might think. It does not mean that the analyst is terrible, it is just a testament to the ubiquitous nature of DNA and how easily it can be transferred. Of course, if this is occurring in laboratories, remedial steps must occur to limit the chances of it happening again.
With that basic information, enjoy the articles!