A few dozen genetic changes can help explain why people of European descent have so many different shades of hair, eye and skin colour - but it is still impossible to tell the colour of someone's eyes or hair based on DNA alone, researchers said on Sunday.
The team at Decode Genetics said their scans of 7 000 Icelandic and Dutch people found 60 separate genetic mutations linked with hair, eye and skin colour.
No single mutation
As with earlier gene surveys, no single mutation or cluster of mutations can tell whether a person has brown, blue or green eyes; brown, blond or red hair or whether his or her skin is fair or freckled.
But, writing in the journal Nature Genetics, Kari Stefansson of Decode Genetics and colleagues said their new suite of genes help narrow down the possibilities and might be used to study certain diseases that are more common in people with certain colouring.
"It has long been thought that before the migrations that first brought our species out of Africa some 60 000 years ago, ancestral human populations had characteristically dark skin, eyes and hair," the researchers wrote.
In Europe, humans evolved big variations in skin, eye and hair colour. But skin and hair tends to be darker in populations originating from near the equator, which supports the idea that pigmentation helps protect from the sun's radiation.
It is unclear why people living in the North - including not only Europeans but many Asians - have lighter skin.
"The most obvious functional advantage of lighter skin pigmentation in northerly latitudes is that it facilitates the synthesis of vitamin D3 in spite of low levels of ultraviolet radiation exposure," the researchers wrote.
The big variations in eye and hair colour among Europeans are even more mysterious.
Stefansson's team found evidence that these variations are heavily selected for - meaning people who have them are more likely to reproduce and pass them along to offspring. This could be because they provide a survival advantage or merely because people like such traits and tend to choose mates who have them.
The researchers found 60 different single nucleotide polymorphisms, or SNPs, associated with red, blond or brown hair; brown, blue or green eyes; freckles and skin sensitivity to sunlight.
SNPs are single-letter changes in the genetic code.
"The 60 SNPs (are) clustered in different genomic regions along five different chromosomes (6, 12, 14, 15 and 16)," they reported.
Most SNPs just predict a tendency to have, for instance, green versus blue eyes or brown versus blue eyes.
"A SNP on chromosome 6p25 is associated with an increased likelihood of freckles and skin sensitivity to sunlight, as well as to brown hair," they added.
Decode, which aims to develop tests to market for various diseases and other uses, said the findings may help "forensic geneticists, and studies of diseases of the skin and eyes that are known to be correlated with such traits." – (Reuters Health)