The Rare Eye Feature That Lets Some People See Colors Others Can't

Have you ever studied a rose only to notice the dozens of subtle color variations? Those without a color impairment can see these hues due to the cones in their eyes, a photoreceptor cell on the retina, per the American Academy of Ophthalmology.

The most common type of color vision is called trichromacy, due to the three types of cones on the retina. For people with trichromacy, about 64% of their cones are L-cones (which see reds), 32% are M-cones (which register greens), and between 2% and 7% are S-cones (which pick up blues), per Hunter Lab. These cones help you see the world's colors, but not everyone has them. Those with color blindness might be missing one or all of the cones for a specific color, making it impossible to see red, green, and blue hues. For example, Pantone notes that many people have been known to exhibit red and green dichromatism (which renders those two colors hard to distinguish). Moreover, approximately 1% of women — and about eight times as many men — suffer from a color impairment.

You can also be on the opposite end of the spectrum and have color enhancements in your vision by being born with extra cones, called tetrachromacy. Learn what it means to be a tetrachromat and just how many colors this rare disorder gives you the ability to see. You'll also find out why it happens more in females and how to determine if you have it.

The colorful world of tetrachromacy

You've probably seen superhumans racing across the screens of your latest Marvel flick. While you might not have the ability to lift semis or scale walls, you can have the ability to see 100 times more colors than your friends if you have tetrachromacy.

According to BBC Science Focus, a person is born with a fourth cone, known as a tetrachromat, and can see new color ranges within the yellow and green spectrum, opening up millions of new color variations. In fact, an ophthalmology professor at the University of Washington, Dr. Jay Neitz, told Healthline that those with the tetrachromacy mutation can see 100 million colors rather than just the one million colors available to trichromats.

In addition to seeing the world in colors that your friends would be jealous of, RMIT University speculates that low light isn't a barrier for tetrachromats since they might be able to see more with their enhanced sensitivity. While this might be extraordinary in humans, there are a number of animals with four cones, like birds and fish, according to a 2014 study in Current Biology.

Tetrachromacy strikes the X chromosome

Tetrachromacy is one of those rare disorders that is only detected in a few individuals, and generally in women. According to 2010 research in the Journal of Vision, the color mutation leading to tetrachromacy is carried by about 12% of women, but it's hard to test.

The reason that it's typically a gene mutation that hits females rather than males is because it's passed through the X chromosome, according to NeuWrite West. Since females are born with an XX and males are born with an XY, they win out when it comes to the superhuman vision race since they can have the tetrachromacy mutation on both X chromosomes, per Healthline. Once they've got the gene mutation, their brain conforms to process this new information, giving them a world of millions of brighter and distinct shades. 

Additionally, Newcastle University notes that since males only carry the gene leading to tetrachromacy on one X chromosome, they can have anomalous trichromacy, leading to slightly different vision than usual, or color blindness. Therefore, a male will have slight color blindness in the red and green spectrum or no red or green color vision at all. 

How to test for tetrachromacy

The only surefire way to know if you carry the gene mutation leading to tetrachromacy is to have a genetic test done for that specific gene, since sometimes the mutation isn't expressed in a way that gives you superhuman color-detecting abilities. However, researchers have found a few ways to determine if you might carry this specific mutation. For example, the Newcastle University Tetrachromacy Project looks for participants with a male relative with a color anomaly in the red and green areas, since they are more likely to pass on the gene.

Additional testing is also completed to test a person's ability to distinguish between two shades of a color. According to Healthline, those with a tetrachromacy gene can accurately detect color variations or similarities every time they look at them, while those without the mutation cannot. You can try out these tests online, but the variations in screens might lead to a false positive; therefore, it's best to get the testing done in a facility that can ensure accurate results.

Why does it matter? Understanding this phenomenon can also have impacts on the world, as stated by Dr. Kristopher Jake Patten in Psychology Today. He told the publication, "Embracing the differences of tetrachromacy could yield similar advances. A few recent studies have shown that people are better than chance at distinguishing between ill and well faces, possibly based on coloring [...] Tetrachromacy could hold the key to such technology."