Ask Prof: What causes eye color? – with lots o’ links!

So some of my Anatomy students just had a practical on the eyes. One inquiring mind asked, if the iris is made up of muscle, are the muscles green in people with green eyes?

As a review, for those who’ve forgotten: The colored part of the eye is the iris and has muscle that controls how big that black middle opening is (the pupil).


The front of the eye has a cornea and lens to bend light so it focuses on the back wall of the eye, where it is supposed to: the macula densa part of the retina (fovea centralis). Between the cornea and the lens is a black hole we can each see in each other’s eyes – the pupil. A series of muscles surround the pupil to regulate the amount of light that goes through – they lie in a colored area called the iris.

So why do some of us have brown, blue, green, or exotic … colored eyes? … and I mean without those colored contacts. (have you seen those cat-eye contacts during Halloween?!)

Eye Color

Eye color is created with the same pigment skin color is created. Remember melanin, created by the melanocytes?

The concentration of melanin in the iris will make the different eye colors. The iris actually has layers – the melanin pigment can be in the back epithelium or in the stroma (front part of the iris). The different places and how much melanin is deposited makes the wide variety of colors possible. Wikipedia adds that the density of cells in the iris also plays a role in eye color.

Melanin is a dark pigment. Those with brown eyes have a lot of melanin deposited in the iris. Because melanin isn’t fully present at birth, many babies have blue eyes at birth that will later change to brown or green.

Nearly all people will have some melanin in the back layer. Blue-eyed people have no melanin in the front layer and melanin in the back layer, which appears blue because of light refraction, the Stanford School of Medicine reports. Brown-eyed people have a lot of melanin in the front layers, while people with green or hazel eyes have less than brown-eyed people but more than blue-eyed people.


You do inherit eye color from your parents, but it’s not as easy to predict as you might remember of Mendelian genetics –

It’s also not caused by a mixture of colors, like mixing paint. It’s a bit more complex than that too. Factors that determine eye color are found on different genes – this is called a polygenic trait. Some of these genes will allow other genes, holding color information, to be read.

Researchers in Europe, though have figured out a way through DNA testing to predict eye color of kids with 90% accuracy –

Another article that says all blue-eyed humans come from a single, common ancestor –


The iris can change color, depending on how much melanin is deposited. This can happen when pupil size changes, when you get emotional, with age (as noted before).

There is also a laser surgery that can turn brown eyes blue – permanently.

Some people have different eye colors in each of their eyes – something called heterochromia iridium – and different colors in the same iris –





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