The Science of Eye Color

The eye is an incredible mechanical marvel and an impressive piece of engineering by nature. It has enabled humans to see and explore the vastness of this universe and all the things in it visible to us. The first thing people usually notice when looking into somebody's eyes is not its engineering but its color. Eye color is an amazing feature of the human body that contributes to the aesthetics and uniqueness of a person. It adds to a person's beauty and we often compliment people as we like the color of their eyes. In this article, we will explore the science behind this eye color.

Firstly, one wonders where this color in the eyes comes from. Well, it arises from the pigmentation in the Iris. Iris is a structure of the eye that surrounds the pupil (a black hole in the center of the eye) and it controls the amount of light that enters the eye. The color of the person's Iris ranges from very light blue to dark brown as the most frequently categorized eye colors are blue, green/hazel, grey, and brown. Most individuals in the world have brown eyes color i-e color of their Iris is brown. The pigment that colors our eyes is melanin. Remember! The same pigment also colors our skin and hair. So likewise, skin and hair color, the eye color are also directly affected by the amount of melanin present in the front layer (stroma) of Iris. Brown eye color is determined by the large amount of this pigment in the Iris, Green/Hazel/Grey due to the moderate amount of pigment, and Blue color arises when the amount of this pigment is much less. Therefore, the blue colors arise not because of the blue pigment but as the light gets scattered through the Iris in a similar way to light scattered through the Earth's atmosphere and gives a blue sky.

Secondly, the genetic background of eye color is important to discuss as most of it is determined by the variation in a person's genes. As many as 150 genes have been discovered that influence eye color. Most of the genes involved in determining a person's eye color are involved in the production, storage, and transport of melanin pigment. Two genes present very close to each other on chromosome 15 play a major role in eye color. These include OCA2 and HERC2. Several other genes that are involved in determining skin and hair coloring are also reported to play minor roles in determining a person's eye color. These include ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR and TYRP1. The effects of these genes are likely to combine with the effect of OCA2 and HERC2 to produce a range of eye colors in different people.

The OCA2 gene produces P protein, which is involved in the maturation of melanosomes. Melanosomes are intracellular organelles in melanocytes (melanin-producing cells) of the skin and eye, where these cells make and store melanin. In this way, P protein plays a major role in the amount and quality of melanin present in the Iris. Variations in the OCA2 gene reduce the production of P protein. The lower amount of P protein produced will mean that less melanin will be present in the first layer of Iris. It shows that people with normal OCA2 gene will have brown eyes but people with several common variations (polymorphism) in the OCA2 gene will have a bluish color of their eyes. In a nearby region on chromosome 15, the HERC2 gene has a segment known as Intron 86 that controls the expression of the OCA2 gene, turning it on or off as needed. At least one variation in this area of the HERC2 gene is reported to decrease the expression of the OCA2 gene and in turn, a lesser amount of P protein will lead to less amount of melanin and lighter color of a person's eyes.

Thirdly, it is interesting to know the inheritance pattern of eye color. Can a child's eye color be predicted by the eye color of his/her parents? The answer is that the inheritance of eye color is much more complex than it was originally thought as multiple genes are involved. Initially, researchers believed for around 100 years that a single gene determined eye color and its inheritance pattern would be simple in which brown eye color was dominant to the blue color and that the parents with blueish eyes would not have a baby with brown eye color. Later researchers disapproved of the model, as it was too simplistic. It is not very common that parents with blue eyes can have a child with brown eye color but this cannot be ruled out as we now know that multiple genes are involved in determining the eye color which makes the inheritance pattern of eye color a complex one. Therefore, yes, the child's eye color can be predicted by the color of the eyes of its parents and near relatives but unexpected results are seen sometimes such as parents with brown eyes can have a baby having blueish eye color and vice versa as polymorphism may arise. Researchers are still studying how different factors cause such variations.

Fourthly, some disorders also affect eye color. For example, Heterochromia is a condition in which an individual has eyes of different colors. It can be caused by genetic variations, anomalies during the development of the eye, as well because of another disease or an injury to the eye. Ocular Albinism is a condition of the eyes in which the pigmentation of the eyes is severely reduced causing very light-colored eyes and problems with vision. Another condition is Oculocutaneous Albinism in which the pigmentation of skin and hair is also reduced in addition to eyes and in turn, the affected individuals will have very fair skin, light-colored eyes, and white or lighter-colored hairs. Ocular Albinism and Oculocutaneous Albinism result from the mutation of genes involved in the production and storage of melanin.

In conclusion, the color of our eyes is a beautiful blend of nature and genetics. It all comes down to the amount of pigment in our irises and the interplay of multiple genes, especially OCA2 and HERC2. While brown is the most common eye color, blue, green, and gray hues arise from fascinating genetic variations. Eye color inheritance can be unpredictable, making every gaze uniquely mesmerizing. In addition, let us not forget, that some conditions like heterochromia add even more intrigue to the story of our eyes. Isn't it amazing how much science is behind that twinkle in our eyes?