Wednesday, 4 April 2012

Tell me the colour of your eyes and I'll tell you why...

by Monica Espinosa Gomez

Eyes can be of many different colours, they’re brown, hazel, green, blue (in their many different shades) and even violet and red! Have you ever wonder why this happens? Is this trait just following the well-known Mendelian path of inheritance, where one trait is dominant over the other? Eyes are the windows of the soul, as many say, so I definitely needed to know what was going on.

Eyes showing the amazing range in colour

After looking for answers, I found this great paper, “Genotype-phenotype associations and human eye colour” by White and Rabago-Smith4. In this paper they clearly state that eye colour depends on the amount and quality of melanin and that genes, particularly two, are responsible for eye colour.
Eye colour depends on the amount and quality of melanin being produced, the more melanin is produced in the melanocyte cells the darker our eyes will be. Looks like melanin is even more important than we thought!

So how is the amount of melanin affected? Here is where genetics steps in. White and Rabago-Smith show in their paper that, as said before, two major genes are involved in eye colour, HERC2 and OCA2. The OCA2 gene codes for a special protein, called P protein, which is in charge of the maturation of melanin, but when the P protein malfunctions changes in eye colour occur. Meanwhile complications in the HERC2 gene also have consequences, like affecting the nervous tissue. The HERC2 gene regulates the expression of the OCA2 gene, so when something changes in HERC2, the expression of OCA2 is altered. For example, when single-nucleotide polymorphism (SNP) occurs in the gene HERC2, the inhibition of OCA2 happens; in this case we can observe a case of epistasis  (one gene altering the other completely).  On the other hand, different SNPs can cause the decreased expression of the OCA2 gene, giving place to incomplete dominance (there is a “mixture” of both expressions but none dominate). This interaction between both genes is very important because eye colour depends on it. 

HERC2 and OCA2 are mostly related with having brown/blue eyes, depending on the SNPs. Meanwhile another gene, MCIR, increases the chance of having green eyes. What about red or violet eyes? This happens when little or no melanin is produced in the melanocyte cells. Red eyes are a result of no melanin (they’re red because of the reflection of the blood vessels in the eyes) and violet eyes occur when there’s little melanin.  Lastly there’s heterochromia, when your eyes either have two different colours in the iris or each one has a different colour, and this is due to mutations that occur in the cells of the iris.

In conclusion, thanks to White and Rabago-Smith’s paper, eyes can be of different colours mainly because of the interaction between two genes HERC2 and OCA2. There are also other genes that increase the chances of having green eyes and finally red and violet eyes are a result of ocular albinism. This information is very important because people can take advantage of it. Check out this video to understand what I mean. 

1. Dickinson, Boonsri. A sampling of the enormous variation in human eye color. N.d. Eye Color Explained, New York, NY, USA. Discover Magazine. Web. 18 Mar. 2012.

2. Klug, W. S. (2006). Conceptos de genética. Madrid: Pearson Education.

3. White, Désirée, and Montserrat Rabago-Smith. "Genotype-Phenotype associations and human eye color." Journal of Human Genetics 56.January 2011 (2011): 5-7. Journal of Human Genetics. Web. 18 Mar. 2012.

4. White, Désirée, and Montserrat Rabago-Smith. "Genotype-phenotype associations and human eye color. | ResearchGate." ResearchGate. N.p., n.d. Web. 19 Mar. 2012. <>.

5. Anonymous. Heterochromia iridis . N.d. Common Iris Cases. Web. 20 Mar. 2012.

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