February 4, 2008
Where Your Blue Eyes Came From
By Michael D. Shaw
Many articles on Health News Digest concern personal characteristics that people are able to change, such as their weight, muscle development, and overall facial appearance. For once, let’s discuss an attribute that you can’t change: Your eye color.
Eye color exists on a continuum from the darkest shades of brown to the lightest shades of blue. Within this continuum are such types as blue with brown spots, brown-green/hazel, gray, and amber. Variation in eye color—most pronounced in Caucasians—is due to differing amounts and distribution of melanin pigment in the iris. Many of us were taught that brown eyes are a “dominant” trait over blue eyes, but simple observation would indicate that there must be more to the story than such a monogenic inheritance model would suggest.
In 1996, geneticist Hans Eiberg, of the University of Copenhagen, was the first to identify the OCA2 gene as being primarily responsible for eye color. Now, in an article just published in Human Genetics, Eiberg and colleagues demonstrate that people with blue eyes have a single, common ancestor. As Professor Eiberg puts it…
“Originally, we all had brown eyes. But a genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a switch, which literally turned off the ability to produce brown eyes.”
It has been known that the OCA2 gene codes for the so-called P protein, and this P protein is involved in the production of melanin, the pigment giving color to our eyes, as well as hair and skin. Clearly, Eiberg’s switch must provide far finer control than simply on/off, otherwise albinism would result. Rather, this switch somehow reduces the melanin production in the iris, “diluting” as Eiberg calls it, brown eyes to blue.
Eiberg’s team analyzed a goodly amount of mitochondrial DNA and compared the eye color of blue-eyed individuals from countries including Jordan, Denmark, and Turkey—representing considerable ethnic diversity. Yet, no matter what the ethnicity, his switch appears on exactly the same spot in their DNA—within the HERC2 gene. From this finding and the point that all blue-eyed individuals have only the smallest degree of variation in the amount of melanin in their eyes, Eiberg concludes that “…all blue-eyed individuals are linked to the same ancestor.”
It is noted that brown-eyed folks exhibit much individual variation as to the DNA locus that controls melanin production.
Mitochondrial DNA was used here because the mitochondria derive entirely from the mother’s egg cell, with no input from the father’s sperm. As such, DNA in the mitochondria has passed down from mother to daughter since the dawn of humanity. By looking at the mutations that have occurred in the mitochondrial DNA, and documenting where those mutations are commonly found, we have a way of tracing our ancestors.
Eiberg references the classic work of L. Luca Cavalli-Sforza et al. entitled The History and Geography of Human Genes to posit that the mutations responsible for the blue eye color most likely originated from the Near East, or the northwest part of the Black Sea region, where the great agricultural migration to northern Europe occurred in the Neolithic period (6,000-10,000 years ago).
As to the preponderance of blue-eyed people in the Scandinavian and Baltic areas, one theory cites the lack of sufficient sunlight to produce Vitamin D. Lighter skin can absorb more sunlight, and the lower melanin to achieve this is reflected in blond and red hair and blue and green eyes. That blue eyes are an apparently survival-neutral manifestation of sundry mutations that improve Vitamin D synthesis, bespeaks the incredible complexity of the human genome.