If you've ever watched CSI, you'll know that suspects frequently leave incriminating traces of their DNA behind at crime scenes. According to a recent study, however, the telltale amount of that genetic material could also point to a person's guilt or innocence.
While we may think of crime-scene DNA as typically being found in bodily fluids such as blood and saliva, a great deal of it takes the form of shedded skin cells which are transferred onto items that a person touches. This material is thus referred to as "touch DNA."
You might assume that the greater the amount of an individual's touch DNA that is found at a crime scene, the more likely that person is to have been involved in the crime. According to research performed at Australia's Flinders University, though, such is not necessarily the case.
Led by Prof. Adrian Linacre, a team of forensic science experts got 100 volunteers of differing ages and sexes to press both of their thumbs onto glass slides at timed intervals (up to a total elapsed time of three hours) after initially washing their hands.
In the process of doing so, each person transferred some of their skin cells onto the glass. Three samples were obtained from each thumb at each interval, with each person doing the test once a day for three consecutive days. By subsequently applying a dye to the samples, the researchers were able to count how many cells each person left on the slide in each test.
First of all, it was found that 98 of the 100 test subjects consistently left the same approximate number of cells on the glass at each interval and on each day.
Furthermore, the scientists determined that based on those numbers, each of the volunteers fell into one of five cell-shedding categories: light, intermediate-light, intermediate, heavy-intermediate, and heavy. This is a finer differentiation than one arrived at in a 2022 study involving just 11 test subjects, who fell into low, intermediate, and heavy cell-shedding categories.
Bearing these new findings in mind, it's possible that defense attorneys could argue that the only reason so much of their client's DNA was found at a crime scene is because that person just happens to be a heavy shedder.
They could likewise argue that their client couldn't have been at the crime scene for the time span stated by the prosecution, because more – or less – of their DNA would have been left behind if they were. And if the DNA itself was an iffy match, the quantity of it could be used to bolster or weaken its connection to a given person.
The data could even be used to eliminate suspects before cases even reach the courts.
"Depending on what type of 'shedder' a person is, confirmation of their shedder status can guide investigators as to the propensity of an individual to pass on their DNA to an item via touch contact with the hand," says Linacre. "This information can be used in a likelihood ratio to evaluate the probability of finding a person’s DNA on an item."
A paper on the research was recently published in the journal Forensic Science International Genetics.
Source: Flinders University via EurekAlert