Beautiful mutants; Blue-eyed cognation
(February 7th 2008) "Gentlemen prefer blondes" and this desirable hair colour is often complemented by sparkling blue eyes. Danish scientists have now discovered that blue eye colour evolved due to a single mutation, uniting all blue-eyed individuals as members of a global family. By Melanie Estrella
"Originally, we all had brown eyes", elucidates Hans Eiberg from the Department of Cellular and Molecular Medicine of the University in Copenhagen, whose team unravelled the origin of blue eyes (Human Genetics 2008, Epub January 3). Although 90 per cent of the world's population are brown-eyed, Northern Europe, and in particular countries bordering the Baltic Sea, boasts an abundance of blue-eyed beings.
By performing linkage and association analyses on the DNA of Danish families with three generations of blue eyes, as well as several non-related blue-eyed Turks and Jordanians, Eiberg and his team detected a mutation that literally switches off the ability to produce brown eyes and gives rise to blue eyes. A mutation responsible for inciting countless odes such as "Blue eyes, Baby's got blue eyes, like a deep blue sea, on a blue, blue day...", as sung devotedly by Elton John.
The mutation, a simple switch from adenine to guanine, is located on a conserved regulatory element of the HERC2 gene, which is positioned adjacent to the gene OAC2. The OAC2 gene is one of the major contributors to human eye colour, as Eiberg discovered about a decade ago. While the function of the OAC2 gene and its product protein-P is still fairly ambiguous, it does appear to be involved in biosynthesis of the pigment melanin. The amount of melanin determines the colour of the iris, as well as that of our hair and skin.
Cell culture experiments in Eiberg`s lab showed the HERC2 gene element controls OAC2 gene activity by inhibiting its promoter. While complete silencing of the OCA2 gene results in total lack of eye, hair and skin colour, known as albinism, decreasing the iris melanin content to just the right amount turns eyes blue.
Since the accordance of analysed HERC2 gene sequences of blue-eyed Danes, Turks and Jordanians was extremely high, Eiberg presumes a common founder mutation caused blue eyes to appear in humans, meaning that all those with blue eyes are, albeit distantly, related.
The Danish eye expert further assumes that blue eyes most likely originated in the Neolithic period, about 6,000 - 10,000 years ago, in the near east area or northwest of the Black Sea - the region from where ancestors of the contemporary Scandinavians and inhabitants of the Baltic States derive.
Having brown or blue eyes does not influence ones chances of survival, in Eiberg's opinion, "It simply shows that nature is constantly shuffling the human genome, creating a genetic cocktail of human chromosomes and trying out different changes as it does so."
But why are blue eyes such a widespread characteristic in Nordic countries? Blue eyes, together with a greater incidence of blonde hair and fair skin, might be indicative of an advantageous higher production of vitamin D, vital in sun-starved countries.
On the other hand, a preference for cobalt blue eyes and blonde hair might be based on nothing more than age-old desire.
