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Genetic Traits

Eye Color and Beyond

Eye color and iris pattern

Human eyes are fascinating. Just take a look at the many different shades of eye color and the intricate, unique iris patterns from all over the world.  The eye color is actually the color of the iris.  In the brown iris there is an abundance of melanocytes and melanin whereas in the blue iris there is much less of them. The amount of melanin in conjunction with white collagen fibers produces different shades of grey, green and hazel.  For example, as light travels through relatively melanin-free iris, collagen fibers within the iris scatter the short blue light to the surface. In fact, iris color exists in continuous shades and color pattern: from the lightest shades of blue to the darkest of brown or black, eyes with different colored ring around pupil, or with two irregular shape color shades.  In general, brown is the most popular eye color, followed by blue and grey.  Green is the rarest eye color. 

In addition to color variations, the fibrous tissue in the iris forms unique pattern in each individual’s eyes.  Just like one’s finger prints, an iris pattern is used for biometric identification.  Some banks now use iris scans, instead of pin codes, as the personal identification method.  Some countries are now using iris scans for border control purpose.  For example, United Arab Emirates (UAE) has been operating an iris scan system since 2001 at all ports of entry to screen for expellees.

Genetic determination of eye colors

It was originally thought that eye color was a simple Mendelian trait, meaning it was determined by a single gene, with brown being dominant and blue recessive.  A Mendelian trait predicts that if both parents have blue eyes, their children will have blue eyes too. However people later found that blue eyed parents sometimes have children with brown eyes.   It is now clear that eye color is a polygenic trait, meaning it is determined by multiple genes.   Among the genes that affect eye color, OCA2 and HERC2 stand out. Both genes are located on human chromosome 15.  OCA2 gene produces a cell membrane transporter of tyrosine, a precursor of melanin.  Mutations in the gene OCA2 result in oculocutaneous albinism, a condition associated with vision problems such as reduced sharpness and increased sensitivity to light.  HERC2 regulates OCA2 genes expression. In the European population, a common polymorphism in HERC2 gene is responsible for blue eye phenotype.  A person who has two copies of C allele at HERC2 rs1293832 will likely have blue eyes. While homozygous TT predicts likely brown eyes.

HERC2, rs12913832 TT Allele CC Allele TC  Allele
Likelihood of eye color for people of European descent 85% chance of brown eyes; 14% chance of green eyes; 1% chance of blue eyes. 72% chance of blue eyes; 27% chance of green eyes; 1% chance of brown eyes. 56% chance of brown eyes; 37% chance of green eyes; 7% chance of blue eyes.

Later on, it was found that a set of six SNPs within OCA2-HERC2, SLC24A4, SLC45A2, TYR and IRF4 genes formed a very effective predictor of eye color, giving successful rate of 93% for brown, 91% for blue and 0.72% for intermediate eye color [1].

Beyond eye color

In addition to eye color, melanin/melanocytes are also responsible for our hair and skin color.  One interesting observation is that only European populations have developed a significant lightening of the pigmentation phenotypes of skin, hair and eye color traits due to genetic selection on multiple genes.  If we consider new lactase persistence allele as an adaptive change in response to availability of diary food (for more discussion on this topic, see genetic trait Lactose Intolerance), then what is the adaptive significance of the light pigmentation in skin and eye?  The reason for light skin color in the European population could be explained by the need for maximal utilization of low level UV light (for Vitamin D absorption) in high latitude European region (see genetic trait skin color).  Genetic studies of different European populations suggested that HERC2 rs1293832 C allele arose from a single founder during last 10,000 years.  There must be a compelling reason for the dominance of the blue eye trait in Europe.  One possible explanation is based on a number of genetic studies linking lighter eye color to the ability to overcome seasonal affective disorder (SAD), a major depressive illness.  As Dr. Richard Sturm suggested “perhaps those with blue eyes may have been able to withstand the dark, depressing days of the Neolithic European winters better than those with brown eye color?” [2] I am sure that future genetics studies will come up with more satisfying and convincing answers --- so stay tuned.

References

[1] Fan Liu et al.  Eye color and the prediction of complex phenotypes from genotypes. Current Biology 19 (5): R192–R193. doi:10.1016/j.cub.2009.01.027

[2] Richard A. Sturm et al. Genetics of human iris colour and patterns. Pigment Cell Melanoma Res. 22; 544–562. doi: 10.1111/j.1755-148X.2009.00606.x

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