How does eye colour work?

How does eye colour work?

It’s often said that the eyes are the windows to the soul, but it’s not often that windows come in a variety of mesmerizing colours. That’s the deal with the human eye, though – just like hair and skin color, various genetic factors combine to produce your unique shade that probably won’t be seen again. While eye colour doesn’t have any strong link to overall visual health, many people prefer the way certain shades look on certain people.

But what’s really going on in the body to produce such wildly differing eye colours? The answer might take you back to high school biology, but after you learn all about how everything from genetics, nature and even certain conditions can cause eye colour to show differently in one person than another, you might be more grateful that you have your shade after all.

A close look at the iris
Before you jump into how eyes end up different hues, it might help to review a little bit of the eye’s internal structures as well as what they’re supposed to do. The part of your eye responsible for colour is called the iris, and while you may focus on how pretty those baby blues look on the guy walking down the street, the iris actually does a much more important job.

The iris surrounds the pupil, the black dot in the center of your eye. If you’ve ever gone from a dark room to a light one with your eyes open, you’ll have an immediate reaction to squint. Not only do your eyelids close, but your iris actually expands to cover up more of the pupil. This lets less light into the eye and keeps you safe from damaging your eyesight.

So the iris is essentially just an organic camera shutter, but that doesn’t explain why it needs to have a whole fleet of colours to dress up in. This is where Stanford University steps in to clear some things up.

First, the iris itself is not responsible for what colour it becomes. Instead, the source explained that it’s really melanin, the pigment that gives your skin and hair its color, that’s behind the different brush strokes you see in everyone’s eyes. The degree of melanin in your eyes also determines the intensity of the colour. Less melanin leaves lighter shades like blue or green, while heavy melanin deposits create dark brown and black eyes.

The gene game
As Stanford University explained, science has a pretty good grasp on what causes eyes to appear in different colours. However, why this happens is another question entirely.

If you don’t happen to remember the day in high school when you learned about Punnett squares – simplified ways to visualize how parents pass genes on to their children – Sewanee University sums it all up pretty well: Both parents pass on two “alleles” to the child. These alleles can be either dominant, meaning they will always produce their intended result if present, or recessive, meaning there needs to be another recessive gene present for that physical look to win out. Roughly speaking, darker eye colours are dominant over lighter shades, so two parents with black eyes can’t have a blue-eyed baby, right?

Wrong. The tricky thing about predicting eye colours is that each parent carries two alleles for the same gene. If you have the darkest eyes in the world, you could just be hiding a recessive gene for blue eyes underneath the black one. If you happen to meet a partner with the same genotype, then there’s a chance you could both pass your recessive blue alleles onto your kids.

Best of both worlds
So now you know that both parents with identically coloured eyes can have babies with irises shaded in ways theirs are not. But what if eye colour went a step further off the rails and gave each eye a different hue?

That’s a condition called heterochromia, which is Latin for “different colors.” People with heterochromia may have one blue eye and one green, or they might merely have different shades of the same colour. Medline Plus explained that infants who show signs of heterochromia may also exhibit symptoms of other congenital conditions, so while it may be beautiful to look at, physicians still need to determine that everything’s OK before you can start ooo-ing and aww-ing at your beautifully quirky newborn.

Some people put a lot of stock in eye colour, while others see it merely as another way the human body is predictably unpredictable. Whatever the causes behind the colour – or colours – of your eyes, you shouldn’t pine your days away wishing your peepers were shaded differently. After all, unless you carry around a mirror with you whereever you go, you spend most of the day not even looking at your own eyes.

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