Though brown is the dominant eye color among humans, there is a great deal of variation in the color of the iris in different populations, from nearly black to pale blue or even to the least common color, green. Individuals with blue eyes make up roughly twenty to forty percent of the European population, and currently about seventeen percent of the American population (though more than half of Americans who were born around the turn of the twentieth century had blue eyes).
The part of the world with the largest percentage of blue-eyed people is the region around the Black Sea and throughout most of Europe, though blue eyes can be seen in varying percentages in individuals around the world. But according to a finding published in the journal Human Genetics, the mutation that causes blue eyes arose in a single individual and spread around the world, meaning that all modern humans who possess blue eyes are related, descended from this one person.
HERC2 and OCA2
Very early in human history, all individuals had brown eyes, as most humans still do today. But approximately 6,000 to 10,000 years ago, probably somewhere around what is now Romania, an individual was born with a particular mutation of the HERC2 gene, which acts with the OCA2 gene to regulate melanin. Melanin is the substance that affects hair, eye and skin color in humans; those born without it manifest albinism.
The mutation that caused the blue eyes in this one individual actually affected the ability of the OCA2 gene to control melanin production. The limited melanin production effectively caused a "dilution" of the brown eye color and rendered the irises blue; complete lack of melanin produces pink irises.
Complicated Eye Color
Human eye color is actually regulated by an entire suite of genes, including EYCL1, EYCL2, and EYCL3, variations of which affect green/blue, brown, and brown/blue eye colors, respectively. The interaction of these genes to produce particular eye colors is tremendously complex, and though most people still have the conception of blue eyes as a simple recessive gene, the fact is that almost any combination of these genes can manifest themselves in a wide array of iris hues.
Blue-Eyed Relatives
By analyzing the mitochondrial DNA of blue-eyed individuals from around the world, researchers were able to determine that all people with blue eyes possess the exact same haplotype with very little genetic variation, and are thus all descended from a single individual. It is still unknown as to why the blue-eyed mutation, having once arisen, spread so far and widely. It's possible the mutation caused other as yet undiscovered benefits unrelated to eye color that encouraged its increase in the gene pool.
Source:
Bryner, Jeanna. "One Common Ancestor Behind Blue Eyes" LiveScience. 31 Jan. 2008. Web. 23 Mar. 2011. www.livescience.com.
Jenny Ashford - Jenny Ashford is a writer and graphic artist from central Florida. Her main area of interest in her Suite 101 articles is science, with a ...
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