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  • Writer's pictureMaria Cholakova

What Influences Eye Color?

Eye color is a complex trait. The influences are multiple genetic and biological factors. Today, let's delve into the key elements that determine eye color, in a simple and understandable way.


Decoding the Genetics Behind Eye Color


1. Melanin and Its Role


Melanin, the primary pigment found in the iris, plays a crucial role in determining eye color. The amount and distribution of melanin within the iris influence whether eyes appear brown, blue, green, or other shades. Melanin is produced by melanocytes, specialized cells within the iris.


High Melanin Levels


Brown eyes have the highest concentration of melanin. It absorbs most of the light entering the eye, giving it a dark appearance.


Intermediate Melanin Levels


Eyes that are green, hazel, or amber have intermediate melanin levels. For example, hazel eyes feature a mix of melanin that reflects light so both brown and green tones are visible. Leading to their distinctive color.


Low Melanin Levels


Blue eyes have a very low melanin content in the iris. It allows more light to scatter within the stroma (a part of the iris), creating blue coloration.


Blue eye

2. Genetic Factors


Eye color is a polygenic characteristic, derived from multiple genes. Science identifies more than 16 genes as contributing to eye color. The most significant being OCA2 and HERC2 on chromosome 15.


  • OCA2: This gene plays a major role in regulating melanin production.

  • HERC2: This gene influences the expression of OCA2. A specific variant of HERC2 can switch off OCA2, resulting in blue eyes.


Other notable genes in eye color determination are TYR, TYRP1, and SLC24A4. They affect melanin production, transport, and storage.


3. Light Scattering and Structural Colors


The structure of the iris also affects eye color through a phenomenon known as Rayleigh scattering. It is the same effect which makes the sky appear blue.


When light enters the eye, it is scattered by the fibers in the stroma. In eyes with low melanin (e.g., blue eyes), this scattering causes the shorter wavelengths of light (blue and violet) to be reflected more than the longer wavelengths. That's giving the iris a blue appearance.


4. Age and Eye Color Changes


Eye color can change over time, especially during infancy. Many babies are born with blue or gray eyes, which can darken as melanin accumulates in the iris over the first few years of life.


  • Newborns: Most Caucasian babies are born with blue eyes, which can darken to brown, green, or hazel over time as melanin production increases.

  • Adults: In rare cases, adults may experience changes in eye color due to hormonal changes, health conditions, or medications.



5. Health Conditions


Certain health conditions and genetic disorders can also influence eye color:


  • Albinism: A condition with a lack of melanin production, leading to very light-colored eyes which may appear red or violet due to the visibility of blood vessels.

  • Heterochromia: A condition where an individual has two different colored eyes. This can be genetic or caused by injury or disease.


By exploring the global eye color statistics and underlying mechanisms eye color, we gain a deeper appreciation for the genetic diversity shaping our world. This knowledge enriches our understanding of human biology while acknowledging individuality.


Checked by Atanas Bogoev, MD.

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