Forensic scientists are a step closer to predicting the colour of a suspect’s hair from their DNA alone after the discovery of more than 100 new genes that influence the shade of a person’s locks.
A test based on the new genetic markers was 10-20% more accurate than existing forensic tests and was most reliable for red or black hair, with brown or blond hair proving harder to predict, researchers said.
“If someone leaves blood at a crime scene, you could say from their DNA whether they had black or red hair with about 90% certainty,” said Tim Spector, a lead author on the work at King’s College London.
The colour of a person’s hair is one of the most heritable features of their appearance, with studies on twins suggesting that genetics explains up to 97% of hair colour. Until now scientists have known of only 13 genes that affect where an individual’s hair will lie on the scale from light to dark.
After studying the DNA of nearly 300,000 people, researchers from King’s and Erasmus MC in Rotterdam uncovered 124 genes that contributed to hair colour, either by directly affecting the production and distribution of the natural pigment melanin or through other biological mechanisms, many of which are still unclear.
The fresh haul of hair colour genes explain 35% of red hair, 25% of blond hair and 26% of black hair, according to the study, reported in the journal Nature Genetics. It is likely that hundreds of other genes also affect hair colour in ways that were too subtle for the scientists to detect.
One factor that complicates a forensic test for hair colour is the common tendency for children who are born blond to have their hair turn brown a handful of years later. While that change itself could well be driven by genes, the researchers found none that seemed to explain the effect. “We know that some blond kids become brown but we have no idea why that is,” said Manfred Kayser, a senior author on the paper at Erasmus MC.
Perhaps more intriguing was the discovery of marked differences in hair colour among the men and women who took part in the study. The scientists, who drew on data gathered by the UK BioBank and the US genetics company 23andMe, found that women were 25% more likely to report having blond hair than men, and three times less likely to say they had dark hair. While the scientists cannot rule out that some participants were mistaken about their own hair colour, the numbers are in line with findings from other studies that used optical instruments to measure hair colour. In the past the differences have been attributed to sexual preferences, with women supposedly preferring darker men and men preferring blond women. “If this was going on, it’s hard to prove,” said Kayser.
Beyond casting light on the biology of human hair colour, the research gives scientists fresh insights into genes linked to diseases such as skin cancer. “Pigments are far more than just cosmetic – they are important for the immune system and play a role in many diseases,” said Spector. “Understanding the genetics could lead to new therapies.”