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What is color blindness, and do I have it?

Color blindness typically involves a decreased or shifted sensitivity to some of the colors of the rainbow. Here’s how it works.

By Joel Hunter, MD

Originally, I was going to try and do a whole routine about how you’d know you might have color blindness (“If youuu... memorized that the top light on a traffic light means stop… you might be color blind”). Then I tried to think of several jokes to fit in that format. I couldn’t do it. In related news, I have a lot more respect for Jeff Foxworthy now. It’s probably best it didn’t work out. Because even though everyone I’ve met who is color blind doesn’t seem like it’s a sensitive subject, a repetitive round of poorly-done ophthalmology jokes about it seems like it would irritate some of them. If you saw the sad list of examples (the one above is the best of them), you’d probably have found it irritating even if you aren’t color blind.

With that fully unnecessary disclosure complete, we can move on to color blindness. The term is accurate, but usually overly descriptive for the actual problem. The number of people who are truly color blind, seeing only on a grayscale, is a fraction of a fraction of a percent. My whole job is seeing people who need to visit the eye doctor and I’ll probably go my whole life without meeting someone who is 100% color blind. In almost all cases, what we refer to as color blindness is much less exotic. It is a decreased sensitivity to some colors. Red and green are the most famous of these.

Color blindness typically involves a decreased or shifted sensitivity to some of the colors of the rainbow. Rainbows always have the same color pattern because white light gets split up into all the colors in the visible light spectrum. Each color is different because light (for the purposes of this discussion) is a wave, and the length of the waves determines its color. If the length—called the wavelength—is long, then the color will be closer to red, which has the longest wavelength in the visible spectrum. If the wavelength is short, then the color will be closer to violet, which has the shortest visible wavelength. We are all color blind to infrared (just a touch longer wavelength than red) and to ultra-violet (just a touch shorter wavelength than violet) because we don’t have photoreceptors able to respond to those colors.

The color blindness we’ve all heard about happens for the same reason. The photoreceptors in our eyes are responsible for sensing red, green, or blue. Those are the only three receptor types we have. If any are impaired in their ability to respond to their corresponding color, it’s called color blindness. The good news here (and the reason we don’t meet people who see only gray) is in nearly 100% of cases, it’s only one of the three colors that’s impaired. It can be because it’s missing (this version is called anopsia) or because it is a bit deformed (called an anomaly). The issue isn’t just for a single photoreceptor though, it is for every photoreceptor of that kind. Since it is a misprint in the DNA, the blueprint for every single red, green, or blue photoreceptor is either misprinted or missing entirely.

Red-green color blindness is the most common by a long shot. It is also about twenty times more likely to happen to a man than a woman. These two facts have the same cause. The genes (blueprints for building every part of you) for both red and green pigment photoreceptors are found on the same chromosome (one of the 46 binders holding those blueprints). The X chromosome happens to hold the genes for both red and green color detection. You may have heard of it because the chromosomes that determine if you’re a man or woman are the big celebrities among the chromosomes. Two X chromosomes: you’re born a girl. One X and one Y chromosome: you’re born a boy. This leads to the red-green dilemma being common and almost always male.

When you’re a guy, you’ve just got that one X chromosome to rely on for every gene it carries. When the red or green photoreceptor gene is busted, you’ve got a problem because that was your only copy. Women, with their two X chromosomes, have a backup copy of those genes on their other X chromosome. It will take care of business and make normal photoreceptors, because that’s what backup blueprints do. This means that a relatively common genetic defect will show up relatively often in men (it’s around 8% of us) and barely ever for women. It happens with red and green because they are unfortunately located on a chromosome with no backup copy in half the population. The blue photoreceptor is way over on chromosome 7 and we’ve all got backups for that one. That’s why you rarely hear about blue color blindness.

To answer the second part of the question about whether you’re color blind, if you’re a woman then you probably aren’t. If you’re a guy, then it may be why certain shades of red or green are hard to decipher. You don’t have to guess since there’s a simple test called an Ishihara test; it can determine if you actually are color blind. It has dots of slightly differing colors with numbers hidden in the patterns. The interpretation of the results says something along the lines of, “if youuu look at Ishihara color patterns and think it looks a Jackson Pollock painting… you might be color blind.”

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