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Colour Blindness: or, How I Was Humiliated By a Rainbow

When I was thirteen years old, something happened. Something terrible and embarrassing, and I’ll never forget it.

School photo

In happier times.

Back in those days, I was a model pupil, a keen and conscientious learner with the good looks to match. But this grade-A student was about to meet his match. Our Arts teacher gave us what seemed at first a relatively straightforward task: paint a rainbow.

I calmly drew on my scientific knowledge. Isaac Newton had famously divided the visible colour spectrum into seven colours, primarily because that just seemed like a good number. The order of their appearance could be remembered using the mnemonic Roy G. Biv. Like so:

Rainbow with labels

Source: Wikimedia Commons

And so I set to work. With my seven colours in hand, I began applying them to the page with the meticulousness of Monet. As I did so, I became aware that a few of my neighbours were suppressing a snigger. I couldn’t understand why. Before long, I was surrounded by a throng of classmates, all pointing and laughing uproariously at the product of my most careful artistic energies. My cheeks blazed scarlet with the embarrassment, and the spiteful laughter echoed in my ears forever and ever.

Welcome to the world of colour blindness.

It turns out I had butchered my rainbow in the most vulgar fashion imaginable: I had confused violet with blue, and I’m not sure where green ended up, and what the hell is indigo anyway?

My sob story is not unique. Around 8% of males (and 0.5% of females) are similarly afflicted with colour blindness – that’s millions of people who, like me, must think twice whenever they’re faced with a colour discrimination task, whether it’s a palette of watercolours or a set of traffic lights.

Traffic light cropped

Yeah, don’t even ask…
Source: Wikimedia Commons

So what is it?

Colour blindness first came to light in 1798, when scientist John Dalton realised, “wait a sec ‒ what is this colour red that people keep talking about?” His brother also had difficulty seeing certain colours, so Dalton rightly inferred that the disorder was hereditary. We now know (based on samples taken from Dalton’s preserved eyeball) that he suffered from deuteronaopia, a less common form of colour blindness.

So that’s the first thing to know: there are many types of colour blindness.

Most of the time, colour blindness is caused by malfunctioning cone cells. These are tiny, tightly-packed cells in the eye that use a pigment called photopsin to distinguish light of different wavelengths (i.e. colours). The three types of cone cell (S-cones, M-cones, and L-cones) each contain a slightly different version of photopsin, and each version specialises in a different range of colours.

Cone_cell_en

The basic structure of a cone cell.
Source: Wikimedia Commons

When you think of colour blindness, you probably imagine living inside a black-and-white TV. This is called monochromacy, and is actually extremely rare (and, no, your dog doesn’t see the world in black-and-white, either). Slightly more common is dichromacy, which occurs when one of the three cone cell types is missing or non-functioning. This is what John Dalton had: his eyes were missing the M-cones that usually absorb green light, making it difficult to distinguish red from green.

Anomalous trichromacy is by far the most common form of colour blindness, and also the basis of my many art-class misfortunes. Like normal people, I still have three types of functioning cone cells, but the colour sensitivity of my photopsin pigments is just a little off.  Red- and green-absorbing cones already overlap in their colour sensitivity, so even a small shift will cause both cone types to absorb a similar spectrum of colours. This makes it difficult for me to distinguish red and green, and all variants therein.

How can I get it?

OK, let’s start right at the beginning. Think back to your earliest moment, when your father’s sperm succeeded in fertilising your mother’s egg.

Sperm-egg

Try not to think about how that sperm got in there.
Source: Wikimedia Commons

Now, both the sperm and the egg were carrying one sex chromosome apiece: the egg contained an X chromosome, and the sperm contained either an X (if you’re female) or a Y (if you’re male).

The most common forms of colour blindness are caused by a recessive mutation on the X-chromosome. Essentially, this means that having one normal X-chromosome is enough to “overrule” a second mutated chromosome. This is great news for women, who have two X-chromosomes, but not so great for men: we receive only a single X-chromosome (from our mothers), so if we’re gifted a faulty X then we’re stuck with it.

This explains why I’m colour blind. My father ‒ a fine gentleman with excellent colour vision ‒ gave me an immaculate Y-chromosome, but both of my mother’s X-chromosomes contained the colour blindness mutation, so I never really had a chance.

Here’s a family tree showing how I ended up with the condition:

Colourblindness genealogy

An illustrated history of colour blindness in my family. Red Xs represent chromosomes with the red-green colour blindness mutation. Females with two red Xs, and males with one red X, are colour blind. Females with only one red X are not colour blind, but still have a 50% chance of passing on the mutation to their offspring.

Truth be told, I’ve exaggerated the extent of my colour blindness troubles. Sure, I’ve worn my fair share of mismatched socks, but it’s usually little more than a day-to-day inconvenience. Australian traffic lights follow a standard vertical layout to remove any ambiguity, and I’ve never been in a situation that required me to defuse a time-bomb. In fact, I would argue that, as far as vision defects go, my short-sightedness is a far greater annoyance.

How can I tell if I’ve got it?

Whenever I inform people that I’m colour blind, their first instinct is to point to the nearest red object and demand, “What colour is this?” Fortunately, science has evolved beyond these cave-man interrogation tactics, and there are now a range of nifty tests for diagnosing colour vision disorders.

The most well-known of these is the Ishihara colour test, in which numbers are hidden within a configuration of coloured dots. People with normal vision can recognise these numbers with relative ease, whereas people like me are forced to squint confusedly into the hodge-podge of tiny dots before shamefully admitting defeat.

Ishihara 2

What you should see: 2
What I see: 2!

Ishihara 6

What you should see: 6
What I see: nothing

Ishihara 42

What you should see: 42
What I see: a very vague “4”

Ishihara 21

What you should see: 74
What I see: 21

Because of differences in lighting and screen resolution, these images are best viewed on paper. Even so, if you recognised all the above numbers, then your red-green colour vision is probably fine.

But don’t look so smug about it. We colour blindees just might be hiding an extra super-power or two.

Andrew Katsis is an MSc candidate in zoology at the University of Melbourne. When he isn’t getting confused about basic colours, he writes about other science that doesn’t quite add up. The image of George “Not Actually My Dad” Clooney was sourced from Wikimedia Commons. All personal images remain the property of Andrew Katsis, and are not to be reproduced without express permission, thanks!

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