by Michael Dean Thompson

With every breath you take, you shed DNA. It is in the skin cells that flake off your body by the millions, the hair that floats off as you walk, and the oil you leave behind on many of the things you touch. And scientists are getting better at capturing discarded DNA and turning it into an identification. The amount of DNA necessary for identification is shrinking and can now be measured in fractions of a nanogram. For a sense of scale, imagine a single grain of sand is divided into a billion pieces; each of those pieces would still be significantly larger than a fraction of a nanogram.

Recent research has shown that people shed DNA at different rates. This led researchers to establish arbitrary categories that classify individuals as either light, intermediate, or heavy shedders. More recent studies have added two more categories at the intersections of the previous three.

While the most recent research examining a larger cohort in Forensic Science International: Genetics has argued that people may exist on a continuum of shedders, there are no real categories. Nevertheless, the variability among shedders creates challenges in understanding how likely a person might be to deposit their own DNA or transfer the DNA of another in a social setting.

A study published in the November 2024 issue of Forensic Science International: Genetics attempts to understand how frequently and reliably the transfer of identifiable DNA occurs in a social setting. Understanding the transfer, persistence, prevalence, and recovery of DNA can help those involved in criminal justice combat misinterpreted genetic evidence gathered from crime scenes. This study was an uncontrolled, activity level evaluation. In it, two homeowners (a female and a male) hosted two visitors (also male and female) whom they did not know beforehand. Likewise, the visitors were not known to each other. The activities of the four subjects were monitored, and their DNA deposits measured. In some cases, the researchers also looked for DNA where no contact had been recorded. The measured DNA was then correlated to the person’s contact history, shedder status, personal habits, and demographics.

DNA Terminology

The study is primarily concerned with forensic use of DNA for the purpose of crime scene identification. As such, it makes use of some specific terms. Because a single DNA strand is a chain of molecules where the critical functions are defined by just four molecules represented as either A, T, C, or G, the DNA is segmented into groups of those molecules for discussion purposes. When considering identification, we are less concerned with the function of the DNA than how often the DNAs change from one individual to another. Approximately 1% of our DNA varies between individuals. The rest is fairly constant, barring mutation.

When a specific segment is known to vary, the segment is called an allele (rhymes with “hill”). Some alleles are segmented into groups called Short Tandem Repeats (“STR”). STRs used for identification purposes come from areas of the DNA not known to code for any genes (though that assertion has been challenged). STRs tend to have common variants into which they can be classified, and those variants are then used to generate probability distributions.

As an example, a variant of the STR THO1 might have a probability distribution of one in five (20%) of appearing in certain populations. We carry 23 pairs of chromosomes (individual strands of DNA), each bearing the DNA of our mother and father. Of the 23 pairs, 22 are non-sex determinant. These “autosomal” DNA are always pairs of X chromosomes. Only the 23rd pair might possess a Y chromosome. If a Y chromosome is present, then the donor is a male; otherwise, the donor is a female. As the Y chromosome is passed down from father to son largely unchanged, STRs collected from the Y chromosome can only be used to exclude a person from an identity match rather than positively establish a match. Similarly, mitochondrial DNA (small amount of DNA found within the mitochondria of cells) is passed down from the mother, though the researchers here did not test for this.

Study Design

The hour-long event took place at the home owned by the hosts. Two weeks prior to the study, the home was cleaned by the hosts, but no further detail about the methods used to clean the home were given. On the morning of the experiment, the participants were instructed to put on a brand new pair of underwear that had been supplied by the researchers. In addition, reference DNA samples were taken from the homeowners, the two visitors, an inhabiting partner of the female visitor, and two co-residents of the male visitor, as well as the researcher on the premises. Additionally, the shedder status of the participants was established by having each place their hand on a glass slide and the amount of DNA left behind measured.

The four participants played a board game as they ate a piece of cake and drank tea. After the board game, the participants were asked to simulate a bathroom trip to urinate where they subsequently washed their hands with soap and water and then dried their hands with a towel the hosts had been using the two previous days. Strangely, the men were asked to touch their penises during the simulation while the women were only asked to sit on the toilet seat. All were asked to flush the commode. The four participants then drank water from new glasses and enjoyed a lollipop. Throughout the experiment, the participants were not permitted to touch each other.

Additional intimate samples were taken five hours after the home visit. At that point, the participants were instructed to place the underwear they had worn for the day into provided envelopes that had been previously sampled. The males were also asked to submit to a penile swabbing and combing of their pubic hair, which they performed themselves. The females of the study, again, were not subjected to similar requirements.

The entire visit was recorded from multiple angles, with the exception of the bathroom visit, so that the researchers could see exactly which items were touched. In addition, the participants were asked to recall any contacts with the items. Some of the experiment items, such as the underwear and other clothing, had multiple samples taken.

The shedder statuses of the participants were determined using roughly the same method one of the lead researchers, M. Goray, had used in a separate study that was published in Forensic Science International: Genetics in 2016. Goray’s earlier work was based on just 10 participants making prints on glass slides with both hands on 12 instances. For this experiment, only three samples—each on a separate day—were taken. However, the female host did not conform to that standard. Instead, her second and third samples were taken on the same day, five and a half hours apart from each other. The two women were categorized as “low” shedders, while the male host and male visitor were “intermediate” and “high” shedders, respectively.

The researchers designed the study to extract DNA samples from certain preselected items, with the participants instructed to contact those items. The video from the experiment provided yet another list of samples to be taken. These items were touched by one or more participants, and some items were not touched at all. In total, 128 samples were taken.

The DNA was amplified (passed through a chemical process that forces the DNA to replicate itself) irrespective the concentration of the DNA present and then quantified. Because of the distinctive Y chromosomes of males, the DNA of the female visitor’s partner, the male visitor and host, the male visitor’s roommates, and any samples suspected of having male DNA were additionally passed through a tool called Yfiler Plus. Those DNA samples were then analyzed against a database of Y chromosome lineages called the Y Chromosome Haplotype Reference Database. It was not explained why the females in the experiment did not likewise have their mitochondrial DNA examined.

Found DNA

The amount of DNA found on the various items varied significantly. It varied from no quantifiable DNA to 46 nanograms (“ng”) with an average of 3.5 ng. The lowest amount the researchers considered to be quantifiable was 0.3 ng. While the items averaged two sources, two items had four contributors. The two four-person samples were collected from the toilet handle in the bathroom and the doorknob from the front door of the male visitor’s home.

Some studies have shown that just by being in the proximity to an item, subjects might spread DNA through casual activities like talking. To control for this, the researchers first cleaned and then took reference samples from some of the selected items. This is presumed to have removed any previously deposited DNA. A vase, which had no direct contact, did not have any quantifiable DNA after the experiment completed. Although the control sample from the vase after the initial cleaning did produce a partial profile, the final test produced no DNA so that, at least in this case, it did not appear nearby conversations shed DNA onto the vase.

With the vase excluded, 127 items remained that had some level of DNA. The tool the researchers used to measure DNA, Quantifier Trio, has a lower bound for accuracy of 0.3 ng. Any measurement below that level must be assumed to be an estimate. Nevertheless, 28 samples measured in at fewer than 0.3 ng, with the lowest at 0.06 ng. For both females and the male visitor, samples from their underwear were among those bearing fewer than 0.3 ng. The extremely low findings were despite instructions to put on the underwear in the morning of the event and to wear them an additional five hours during normal daily life after the event.

The low values were samples from several parts of the underwear but not all. On average, the underwear parts other than the gusset produced 1.2 ng samples for the female visitor and 0.25 ng for the female host. The male host’s underwear averaged 2.8 ng and 0.09 ng for the male visitor. The female visitor’s gusset, however, produced 45 ng.

Twenty-two samples from the female participants were passed through Yfiler Plus. Four samples did not produce any evidence of Y chromosomes. Only three of the remaining 18 samples yielded a full single male source while nine produced a partial single source, with the remaining six being two person samples. Five of the six samples could not be separated into major and minor contributors.

Typically, one contributor would dominate the sample, allowing labs to segregate alleles based on relative weights of the contributions. When the contributor levels are statistically equal, they cannot be uniquely identified using standard techniques. However, some tools that were not used by the researchers have claimed to provide solutions using advanced, controversial statistical analyses. Despite the inability to segregate the samples, the researchers examined the alleles in order to exclude known males. This technique excluded the male visitor from all intimate female samples.

The female visitor had some of the more interesting male DNA findings. Ten of the samples collected from her hands and underpants did not exclude her male partner’s DNA based on the Y chromosome. Except, when the researchers used the same samples to perform an autosomal STR analyses, four of those samples did not indicate his DNA. This might be explained using the gusset of her underwear as an example. It possessed 45 ng, a large amount of DNA. While Y chromosomes similar to that of her partner were found (“strong inclusory support”) in the gusset, his autosomal DNA was not found. The researchers suggest that the large presence of her own DNA overwhelmed the partner’s DNA during autosomal profiling. Essentially, his DNA seemingly faded into the statistical background. An additional unidentified male was also detected in 9 of the 10 Y chromosome samples.

The male host and visitor were found to be intermediate and high shedders, respectively, while the females were both found to be low shedders. For both males, hand and fingernail samples either could only be attributed to the person from whom it was collected or that person contributed more than 95% of the DNA. The female visitor likewise contributed between 91% and 100% of the DNA of her hand and fingernail samples. The female host, however, did not show in one of her own samples. The female host’s left hand gave a sample containing DNA of the male host and an unknown donor. In addition, her left fingernails also produced a single source unknown profile. Keeping in mind that the participants were asked to simulate a bathroom visit where they washed their hands and that the video does not show her touching anyone other than herself after the bathroom visit, the researchers assume the male host DNA she donated must have come from an indirect transfer, and both of the items she contacted after the bathroom visit contained the male host’s DNA.

Unknown DNA was detected in 45 percent of the fingernail and hand samples. In all but one of those samples, the unknown source was a minor contributor, averaging 9% of the DNA sample. Interestingly, the researcher present during the experiment could not be excluded from one of the male host’s most intimate samples, which implies contamination during sample processing.

Indirect Transfer

Nine of the 128 samples taken during the experiment showed signs of indirect transfer. Not only was the female visitor’s partner found on her underwear samples despite not having contacted them, his DNA was also found on a mug she touched for the first time eight minutes into the experiment. This was surprisingly after having made 48 other contacts. Because her partner was found in her hand and fingernail samples, as well as other body parts, they may have acted as the vector that transmitted the DNA.

The female visitor was also found on the male visitor’s glass though she never contacted it. When the researchers checked the tapes, they did find that there were times the male visitor touched items immediately after her, including a jug, jar, and bathroom towel. It is possible his hands picked up her DNA then and transferred it to the glass, yet her DNA was not found on his hands later.

The researchers postulate that her DNA may have been lost from his hands during the two subsequent contacts with his phone or that his status as a high shedder may have masked her (low shedder) DNA. The problem with the latter explanation would seem to be that, as a low shedder, she still emitted enough DNA in the earlier contacts to overcome the difference in shedder status and transfer in sufficient quantity to be recognizable on the glass.

The DNA of an uninvolved party, a relative of one of the hosts, was also found on the female visitor’s pants. The researchers found the same person’s DNA as a major contributor on the chair on which she sat, offering a likely passive transfer vector.

This study applied unequal standards for the male and female participants. The males were subjected to profiling that was more intrusive physically and personally (the Y Haplotype Research Database is an ancestral search along the male line). It was also a very small experiment that would be difficult to replicate. Nevertheless, it illustrated some of the challenges and pitfalls of treating DNA as a fingerprint.

Studies such as this one highlight the sometimes circuitous and highly unexpected transmission of DNA. Unlike a fingerprint, a person need not be present at a location to have their DNA found there or need not have touched an object to have their DNA on it. This was made abundantly clear by the DNA of the female visitor’s partner being found on a mug, despite the efforts of the research designers to eliminate direct contact transference. And, the female visitor’s low shedder status did not prevent her DNA from finding a home on a glass she never touched that was used by a man she did not know and likewise never touched. The implications for those ensnared in the criminal justice system based on DNA evidence are potentially monumental.

DNA is far more (and under certain circumstances, far less) than a simple fingerprint. It identifies us as unique among the billions of people who exist today, have existed in the past, and are yet to exist. It can give hints to our skin color, tell our disease profile, and even suggest an IQ. Yet, the law enforcement is treating shed DNA as if it is a piece of luggage abandoned in an airport. Applying the abandonment doctrine to DNA allows them to search, catalog, and store it with little oversight. DNA, however, is not something left behind by volition, carelessness, or forgetfulness. It is simply left behind and maybe transported elsewhere by others inadvertently.

The study highlights the urgent need for investigators who find trace amounts of DNA at a crime scene to make absolutely certain that all possible innocent vectors for deposit are adequately explored and resolved before using that DNA as irrefutable proof of guilt. Unfortunately, that simply is not the reality in the real world where many in law enforcement still labor under the thoroughly discredited belief that DNA at a crime scene or on a murder weapon necessarily equates to guilt. 

Source: Where did it go? A study of DNA transfer in a social setting, Cahill, Amy et al. Forensic Science International: Genetics, Volume 73, 103101

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