Original photo by Chetty Thomas/ Shutterstock
Human eyes are entirely unique; just like fingerprints, no two sets are alike. But some genetic anomalies create especially unlikely “windows” to the world — like gray eyes. Eye experts once believed that human eyes could appear in only three colors: brown, blue, and green, sometimes with hazel or amber added. More recently, the ashy hue that was once lumped into the blue category has been regrouped as its own, albeit rarely seen, color. Brown-eyed folks are in good company, with up to 80% of the global population sporting the shade, while blue eyes are the second most common hue. Traditionally, green was considered the least common eye color, though researchers now say gray is the most rare, with less than 1% of the population seeing through steel-colored eyes.
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It's a factSun-kissed skin is often dotted with freckles — which can also appear on our eyes. Optical freckles are common and generally harmless; some form before birth as mole-like spots called nevi, while others appear on the iris thanks to sun exposure and aging.
Eye color is an inherited trait, meaning it’s likely members of the same family have similar eye colors. However, geneticists now believe determining a child’s eye color isn’t as simple as looking at their parents. That’s because as many as 16 genes work together to impact the final hue. Intriguingly, the eye color we have at birth isn’t necessarily the one we’ll have as adults. Most babies are born with fainter eyes that often look gray, light blue, or light brown until the melanocytes — the protein that creates color — produce enough melanin to color the iris. People with less active melanocytes typically have lighter eyes (like blue or green), while people with more melanin usually end up with brown eyes. In most cases, our final eye color begins to emerge around 3 to 6 months old, though it can continue changing until a baby’s third birthday.
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Having two different colored eyes is called ______.
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Having two different colored eyes is called heterochromia.
The letters on an eye exam chart are called “optotypes.”
Picking out which letters you can (and can’t) see from a chart is now a routine part of an eye exam, in part due to Dutch ophthalmologist Herman Snellen. For hundreds of years, eye doctors used a variety of methods to test their patients’ visual acuity (aka how far and clearly a person can see), including vision charts of their own design featuring seeds and common symbols, though no one test was widely used. In the 1860s, Snellen designed his first vision chart using squares and circles, but ultimately decided to use letters. The chart-topping sizable E, along with the C, D, F, L, O, P, T, and Z were dubbed “optotypes” — a style of consistently sized and geometrically balanced lettering. Snellen’s test became popular when the British Army began using it around 1863, and it eventually became the standard acuity test. While other charts have since emerged (along with tweaks to Snellen’s design), it remains the most widespread eye exam tool, in part because it’s easy and inexpensive to reproduce.
Nicole Garner Meeker is a writer and editor based in St. Louis. Her history, nature, and food stories have also appeared at Mental Floss and Better Report.
