The Limitations of Secondary Transfer DNA Evidence in Louisiana Criminal Cases

Louisiana criminal cases, especially serious felonies such as homicides, rapes, armed robberies, and burglaries, often involve the use of DNA science. We’ve all heard the term “DNA”, but what does it mean? DNA is an acronym that stands for “Deoxyribonucleic Acid.” DNA is a biological polymer that can be transferred in heredity from one organism to another. Almost every cell in the human body has DNA present. One can think of DNA as being an instruction book for telling cells how to function, grow, replicate, etc. Although DNA science is incredibly complex and rapidly evolving as a subject matter, for purposes of this blog article, it is enough for the reader to understand that DNA is a hereditary material that is contained within almost every cell of an individual. 

Because of its hereditary properties, DNA is remarkably useful in assisting forensic scientists in criminal investigations. DNA can be collected from blood, hair, skin, semen, etc. The power of DNA lies in its ability to show physical contact between a given person’s DNA and an object or scene. Although DNA is an incredibly useful investigative tool, it has serious limitations that are often not well understood or known by laypersons unfamiliar with DNA science. 

​A simple example will suffice to demonstrate just one limitation of DNA. Many other limitations exist, but for now, one limitation–DNA transfer–will be discussed. Consider the simplified hypothetical that follows. You are Person A. Your hands contain skin cells, and each of those skin cells contain DNA within their nuclei. You are sweating after a long workout. While you were working out, the skin on your hands got abraded somewhat from tight gripping of the metal bar of the weights you were deadlifting. These abraded skin cells are barely hanging on to your hand and are ready to slough off at any moment, given the right circumstances. Right after your workout, your hands are a rich repository of your DNA, particularly within your sweat and skin cells. 

You finish your workout and shake hands with Person B. In the process of shaking hands, you transfer your sweat and skin cells to Person B. This action is referred to as primary or direct transfer. Person A’s DNA is directly transferred to Person B during the handshake. Person B now has Person A’s DNA on his hand. Person B does not wipe his hands, wash his hands, or take any other action to rid himself of Person A’s DNA. 

Person B leaves the presence of Person A. Within a half hour, Person B is somewhere else. Person B takes the same hand that was used to shake Person A’s hand and grabs a pistol out of the glovebox of his car. In grabbing the pistol, Person B has now transferred some of Person A’s sweat and skin cells to the grip and trigger of the pistol. This type of transfer is known as secondary transfer of DNA. 

Person B takes this same gun and then shoots and kills someone in a drive-by shooting. No one else sees Person B shoot. Person B tosses the gun to the ground. Law enforcement later arrive at the scene, recover the gun, swab it for DNA, and retrieve DNA from the gun. Shell casings recovered from the crime scene are also correlated with the barrel of the pistol.
 It turns out that Person A has a prior felony conviction for burglary, and Person A previously had to give a DNA sample when he was booked into jail years ago for the felony offense. Getting back to the hypothetical case, when law enforcement enter the unknown DNA from the gun into CODIS (Combined DNA Index System), the unknown DNA from the gun matches the DNA from Person A’s felony burglary conviction case. Law enforcement now believe that they know who the killer is. They believe Person A is the killer. 

No other DNA was recovered from the gun in a quantifiable amount, not even Person B’s DNA. Assume Person B’s DNA was on the gun since he was the actual killer, but the amount of DNA deposited by Person B is not quantifiable. In forensic science, if an insufficient amount of DNA is not deposited, when an analyst later arrives at a crime scene and swabs for possible DNA evidence, the amount picked up on the swab may not later be enough to biochemically amplify at a laboratory during the Polymerase Chain Reaction (afterwards, PCR) process. The PCR process is one stage of several stages that is necessary to “match” an unknown DNA profile to a known DNA profile. 

In this hypothetical, we know who the real killer is. The real killer is Person B. However, secondary transfer of DNA in this hypothetical makes it seem as if Person A is the killer when he is not only because Person A’s DNA was left behind in a quantifiable amount and Person B’s DNA was not. Why did Person B’s DNA not also transfer to the gun in a quantifiable amount? Shouldn’t there have been at least direct transfer of a sufficient amount of Person B’s DNA to the gun? An infinite number of variables can influence whether, and how much, DNA is transferred. Heat, light, grip pressure, angles, genetics, etc. can all influence whether, and how much, DNA is transferred. Some people even regularly shed more skin cells than others or sweat more than others, thereby leaving behind more potential DNA. For the purposes of this hypothetical, let’s assume that, for whatever reason, Person B’s DNA did not transfer to the gun in a quantifiable amount even though Person B pulled the trigger and actually touched the gun. We now have a situation in which Person A, an innocent person, can be brought into the criminal justice system because of an inherent limitation in DNA science that arises during secondary transfer. This risk is heightened if the law enforcement agency investigating the case misunderstands or misapplies the DNA results in this hypothetical. Innocent Person A could be charged with murder because it looks as if he is the killer when he is not. 

As the previous hypothetical just demonstrated, when encountering a Louisiana criminal case involving DNA science, it’s important to hire a competent legal professional who can meticulously examine all of the evidence to ensure that you are not wrongfully convicted of a crime. DNA evidence is much more limited in its usefulness than many people realize, and a skilled legal practitioner can expose its limitations to a judge and jury. Not all attorneys understand the science of DNA. If you are factually innocent and facing charges based on a misunderstanding or misapplication of DNA science by law enforcement, reach out to a skilled advocate who can fight to demonstrate your innocence so that you don’t become a statistic in the legal system.