Have Blue Eyes? Then We're Related.

Credit: iStockphoto/Cristian Ardelean

Variation in the color of the eyes from brown to green
can all be explained by the amount of melanin in the iris,
but blue-eyed individuals only have a small degree of
variation in the amount of melanin in their eyes. 

There are days when I just scan through random research reports from past years, and find things that make me say, "Well, this is certainly interesting." 

If you have blue eyes as I do, then we share a common ancestor who lived between six and ten thousand years ago.  Not ancestors, plural, but an ancestor, singular, that had a random mutation in a single gene that created the first blue-eyed person ever. Until about ten thousand years ago, everyone of the planet had black skin with eyes that varied from brown to green, but starting with this one person, there were blue eyes.  Meaningless in most respects, but interesting in that it shows the power of one mutation in one gene in one person.

Enough to make most blue eyed folks say, "Wow.  Way cool."

So let's start a club, the Blue Eyed Gene Pool.

Here are the results of several studies into the genetics of blue eyes, with links to original research in the attribution line.
*  *  *  *  *

Blue-eyed humans have a single, common ancestor
Research shows that people with blue eyes have a single, common ancestor. A team at the University of Copenhagen have tracked down a genetic mutation which took place 6-10,000 years ago and is the cause of the eye color of all blue-eyed humans alive on the planet today.

What is the genetic mutation
"Originally, we all had brown eyes," said Professor Hans Eiberg from the Department of Cellular and Molecular Medicine. "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."

The OCA2 gene codes for the so-called P protein, which is involved in the production of melanin, the pigment that gives color to our hair, eyes and skin. The "switch," which is located in the gene adjacent to OCA2 does not, however, turn off the gene entirely, but rather limits its action to reducing the production of melanin in the iris -- effectively "diluting" brown eyes to blue. The switch's effect on OCA2 is very specific therefore. If the OCA2 gene had been completely destroyed or turned off, human beings would be without melanin in their hair, eyes or skin color -- a condition known as albinism.

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Limited genetic variation
Variation in the color of the eyes from brown to green can all be explained by the amount of melanin in the iris, but blue-eyed individuals only have a small degree of variation in the amount of melanin in their eyes. "From this we can conclude that all blue-eyed individuals are linked to the same ancestor," says Professor Eiberg. "They have all inherited the same switch at exactly the same spot in their DNA." Brown-eyed individuals, by contrast, have considerable individual variation in the area of their DNA that controls melanin production.

Researchers examined mitochondrial DNA and compared the eye color of blue-eyed individuals in countries as diverse as Jordan, Denmark and Turkey.

Nature shuffles our genes
The mutation of brown eyes to blue represents neither a positive nor a negative mutation. It is one of several mutations such as hair color, baldness, freckles and beauty spots, which neither increases nor reduces a human's chance of survival. As Professor Eiberg says, "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."

Eye color also may be an indicator of whether a person is high-risk for certain serious skin conditions. A new study shows people with blue eyes are less likely to have vitiligo, an an autoimmune skin disease in which pigment loss results in irregular white patches of skin and hair. The flip side is that those of us with blue eyes may be susceptible to melanoma, the most dangerous kind of skin cancer.

Blue eyed men prefer blue eyed women
According to Bruno Laeng and colleagues, from the University of Tromso, Norway, blue eyed men prefer blue eyed women, perhaps because it is a way to ensure he is the father of any children of the union.  Here's how that works.  The laws of genetics state that eye color is inherited as follows:
  1. If both parents have blue eyes, the children will have blue eyes.
  2. The brown eye form of the eye color gene (or allele) is dominant, whereas the blue eye allele is recessive.
  3. If both parents have brown eyes yet carry the allele for blue eyes,  a quarter of the children will have blue eyes, and three quarters will have brown eyes.
It then follows that if a child born to two blue-eyed parents does not have blue eyes, then the blue-eyed father is not the biological father. It is therefore reasonable to expect that a man would be more attracted towards a woman displaying a trait that increases his paternal confidence, and the likelihood that he could uncover his partner’s sexual infidelity.

Both blue-eyed and brown-eyed women showed no difference in their preferences for male models of either eye color.  Similarly, brown-eyed men showed no preference for either blue-eyed or brown-eyed female models.  However, blue-eyed men rated blue-eyed female models as more attractive than brown-eyed models.

Eye color may be linked to alcohol dependence
In a separate study, researchers from the University of Vermont recently noticed a strange correlation: people with blue eyes might have a greater chance of becoming alcoholics.

The authors in this study found that European Americans with light-colored eyes -- including green, grey and brown in the center -- had a higher incidence of alcohol dependency than those with dark brown eyes, with the strongest tendency among blue-eyed individuals. The study outlines the genetic components that determine eye color and shows that they line up along the same chromosome as the genes related to excessive alcohol use.

From an extensive database, researchers filtered out the alcohol-dependent patients with European ancestry, a total of 1,263 samples. After noticing the eye-color connection, researchers retested their analysis three times, arranging and rearranging the groups to compare age, gender and different ethnic or geographic backgrounds, such as southern and northern parts of the continent yet arrived at the same result.

Related stories:
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  1. Materials provided by University of Copenhagen. Hans Eiberg, Jesper Troelsen, Mette Nielsen, Annemette Mikkelsen, Jonas Mengel-From, Klaus W. Kjaer, Lars Hansen. Blue eye color in humans may be caused by a perfectly associated founder mutation in a regulatory element located within the HERC2 gene inhibiting OCA2 expression. Human Genetics, 2008
  2. Materials provided by University of Vermont. Arvis Sulovari, Henry R. Kranzler, Lindsay A. Farrer, Joel Gelernter, Dawei Li. Eye color: A potential indicator of alcohol dependence risk in European Americans. American Journal of Medical Genetics Part B: Neuropsychiatric Genetics, 2015
  3. Materials provided by Springer. "Blue Eyes -- A Clue To Paternity." ScienceDaily, 23 October 2006.
  4. Materials provided by NIH/National Human Genome Research Institute. Christian Praetorius, Christine Grill, Simon N. Stacey, Alexander M. Metcalf, David U. Gorkin, Kathleen C. Robinson, Eric Van Otterloo, Reuben S.Q. Kim, Kristin Bergsteinsdottir, Margret H. Ogmundsdottir et al. A Polymorphism in IRF4 Affects Human Pigmentation through a Tyrosinase-Dependent MITF/TFAP2A Pathway. Cell, 21 November 2013.
  5. Materials provided by University of Colorado Denver. Ying Jin, Stanca A Birlea, Pamela R Fain, Tracey M Ferrara, Songtao Ben, Sheri L Riccardi, Joanne B Cole, Katherine Gowan, Paulene J Holland, Dorothy C Bennett, Rosalie M Luiten, Albert Wolkerstorfer, J P Wietze van der Veen, Anke Hartmann, Saskia Eichner, Gerold Schuler, Nanja van Geel, Jo Lambert, E Helen Kemp, David J Gawkrodger, Anthony P Weetman, Alain Taïeb, Thomas Jouary, Khaled Ezzedine, Margaret R Wallace, Wayne T McCormack, Mauro Picardo, Giovanni Leone, Andreas Overbeck, Nanette B Silverberg, Richard A Spritz. Genome-wide association analyses identify 13 new susceptibility loci for generalized vitiligo. Nature Genetics, 2012

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