A new system developed by Dutch, American, and Polish forensic scientists combines genetic testing and computer modeling to create an accurate “sketch” of a individual’s coloration – a breakthrough that will enable detectives to build profiles of unidentified criminals, and archaeologists to reveal characteristics of discovered human remains.
First introduced to the public in the early 2010s, genetic testing protocols known as forensic DNA phenotyping (FDP) can predict an individual’s hair and eye color based on analysis of trace amounts of DNA left at crime scenes. The nascent field was initiated so that investigators gather information about suspects and victims in instances where no reference DNA is available for comparison.
One FDP platform known for its high level of accuracy is the HIrisPlex, created by researchers at the Erasmus MC University Medical Centre in the Netherlands in 2014. The system first uses a DNA assay to look for specific genetic variations, called SNPs, within genes associated with hair and eye pigmentation. The results can then entered into a free online tool that calculates how the SNPs interact to generate a person’s most likely real-life coloration. Because the model is open-access, any lab with SNP test data can utilize it.
Though other systems claim to also predict characteristics such as face shape or height, the results must be taken with a grain of salt because these features are mediated by complex interactions in many genes, whereas the ones that determine brown hair vs blonde are fairly straightforward.
And until now, FDP methods for determining skin color were also unreliable.
As described in the journal Forensic Science International: Genetics, the team behind HIrisPlex has demonstrated that their upgraded system, HIrisPlex-S, is the FDP capable of simultaneously detecting hair, eye, and skin color from DNA samples.
"We have previously provided law enforcement and anthropologists with DNA tools for eye color and for combined eye and hair color, but skin color has been more difficult," study co-director Susan Walsh said in a statement.
"Importantly, we are directly predicting actual skin color divided into five subtypes -- very pale, pale, intermediate, dark and dark to black -- using DNA markers from the genes that determine an individual's skin coloration. This is not the same as identifying genetic ancestry. You might say it's more similar to specifying a paint color in a hardware store rather than denoting race or ethnicity”
The HIrisPlex-S assay assesses DNA for the presence of 41 total SNPs: 17 SNPs associated with skin coloration and the 24 iris and hair pigmentation SNPs used in HIrisPlex, 19 of which also contribute to skin color. The presence or absence of the SNPs are then entered into a new version of the computer model.
During validation experiments, Walsh and her colleagues showed that the system was suitable for analyzing “simulated forensic casework samples such as blood, semen, saliva stains, of inhibited DNA samples, of low quantity touch (trace) DNA samples, and of artificially degraded DNA samples.”
Walsh hopes that the improved protocol will help law enforcement to track suspects without relying on the inherently unreliable and biased accounts of witnesses.
"If anyone asks an eyewitness what they saw, the majority of time they mention hair color and skin color. What we are doing is using genetics to take an objective look at what they saw," Walsh said.