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Understanding Eye Color

eyes green close up woman

People always ask Dr. Beth and me about the color of their eyes. Are they brown? Hazel? Green? Well, it is complicated. Eye color is a hereditary trait that depends on the genes of both parents, as well as a little bit of mystery. The color of the eye is based on the pigments in the iris, which is a colored ring of muscle located at the center of the eye (around the pupil) that helps to control the amount of light that comes into your eye. Eye color falls on a spectrum of color that can range from dark brown, to gray, to green, to blue, with a whole lot of variation in between.


The genetics of eye color are anything but straightforward. In fact children are often born with a different eye color than either of their parents. For some time the belief was that two blue-eyed parents could not have a brown-eyed child, however, while it’s not common, this combination can and does occur. Genetic research in regards to eye color is an ongoing pursuit and while they have identified certain genes that play a role, researchers still do not know exactly how many genes are involved and to what extent each gene affects the final eye color.

The Iris

Looking at it simply, the color of the eye is based on the amount of the pigment melanin located in the iris. Large amounts of melanin result in brown eyes, while blue eyes result from smaller amounts of the pigment. This is why babies that are born with blue eyes (who often have smaller amounts of melanin until they are about a year old) often experience a darkening of their eye color as they grow and develop more melanin in the iris. In adults across the globe, the most common eye color worldwide is brown, while lighter colors such as blue, green and hazel are found predominantly in the Caucasian population.

Abnormal Eye Color

Sometimes the color of a person’s eyes are not normal. Here are some interesting causes of this phenomenon.

Heterochromia, for example, is a condition in which the two eyes are different colors, or part of one eye is a different color. This can be caused by genetic inconsistencies, issues that occur during the development of the eye, or acquired later in life due to an injury or disease.

Ocular albinism is a condition in which the eye is a very light color due to low levels of pigmentation in the iris, which is the result of a genetic mutation. It is usually accompanied by serious vision problems. Oculocutaneous albinism is a similar mutation in the body’s ability to produce and store melanin that affects skin and hair color in addition to the eyes.

Eye color can also be affected by certain medications. For example, a certain glaucoma eye drop is known to darken light irises to brown, as well as lengthen and darken eyelashes.

Eye Color – It’s More Than Meets the Eye

It is known that light eyes are more sensitive to light, which is why it might be hard for someone with blue or green eyes to go out into the sun without sunglasses. Light eyes have also shown to be a risk factor for certain conditions including age-related macular degeneration (AMD).

Color Contact Lenses

While we can’t pick our eye color, we can always play around with different looks using colored contact lenses. Just be sure that you get a proper prescription for any contact lenses, including cosmetic colored lenses, from an eye doctor! Wearing contact lenses that were obtained without a prescription could be dangerous to your eyes and your vision.

-Dr. Nate




Why Do Some People Have Blue Eyes ?

People are fascinated by eye color. When I talk to expectant parents, it’s fun to talk about the likelihood of their baby having brown, blue, or green eyes. When Nora was born, she, like many infants, had grayish-colored eyes. Some thought her eyes were blue, but I was pretty sure they would be brown like her mother’s.

What causes specific people to have different eye color? Well, we’ve known for a long time that eye color is genetically based. Recently, however, a team at the University of Copenhagen team has identified the specific gene that determines if someone’s eyes are going to be blue.

Professor Eiberg from the Department of Cellular and Molecular Medicine explained that “originally, we all had brown eyes. 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.” It is estimated that this mutation occurred 6,000 to 10,000 years ago.

People who have brown eyes have a lot of a pigment called melanin in the iris. (This is the same pigment that gives some people darker skin color than others.) People with green eyes have less melanin in their irises than brown-eyed people, and people with blue eyes have the least amount of melanin.

Evidence now shows that all blue-eyed people have a common ancestor. The genes that control eye color have significant variation, but the gene responsible for blue eyes is much more specific. In the group studied, blue-eyed people had all inherited the same switch at exactly the same spot in their DNA.

This doesn’t mean that anyone has any more control over whether their children are going to have blue eyes or not, but we do have a better understanding of why I have blue eyes, but my daughter does not.

Be Well!

Dr. Nate

Nathan Bonilla-Warford, OD
Bright Eyes Family Vision Care



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