The Grass is Green

 The grass is green.

 The sky is blue.

 The sun is yellow.

It is as simple as that.

Actually, it is not.


Have you ever thought about how other people perceive colours? Or how animals do? When you were a child, you were probably told that animals see black and white. Well, most of them do not. But their vision is often very different from ours.


We, humans, are able to perceive three basic colours (that make up all the colours we see) - or three wavelengths, to be more specific: short (blue), middle (yellow/green) and long (red). This ability is unique to us and a specific group of primates called the “old world monkeys”, which are our close relatives.


Vision varies throughout the animal kingdom. For example, simple vertebrates only see black and white - this is called monochromatic vision (only one colour is perceived).


Most mammals can perceive two wavelengths: middle and short, which means they cannot see the colour red. This is called dichromatic vision. This applies to dogs, cats, rodents, new world monkeys, etc. However, what is important is that many of these mammals can see a part of the ultraviolet spectrum. For example, rodent urine reflects UV rays so the ability to perceive them is very useful for them when marking their territory.


Then there are humans plus the old world monkeys with the “trichromatic” vision (three colours).


Now you might ask: where are the birds and the reptiles? Is it possible for an animal to be able to perceive more colours than we do? Well, yes it is. Birds and reptiles have what is called tetrachromatic vision (four basic colours), which makes them able to perceive far greater amount of colours than most humans can.


Wait, did I say MOST humans?


Well, I did.


There are, in fact, humans with tetrachromatic vision, too. The same as humans with dichromatic vision which is caused by certain changes in the genetic information you carry. You see, the colours your eye perceives are transmitted to your brain by proteins called “opsines” which are coded by your genes. There is one specific opsine for each of the three basic colours you see, so when the genetic information malfunctions, the opsine is not coded properly and the colour is not transmitted into your brain, where it cannot be made into the picture you should see.


This is how deuteranopia and protanopia (which are both dichromatic disorders with just a little bit different error in the middle-wavelength opsine) work. The flawed opsines, however, are not completely useless; they just make you see a slightly different part of the colour spectrum than you normally should.


Thus if a woman with protanopia has a daughter with a deuteranopic man (or the other way round), she will inherit one flawed version of the middle-wavelenth opsine-coding gene from each parent, which means that she will have four different opsines instead of three and she will see unimaginably more colours than any regular human being.


Now, why did I say “daughter”? Am I trying to discriminate men?


No I’m not. The truth is that the genes coding the middle and the long-wavelength opsines can both be found on the X chromosome, and since men only have one X chromosome (and one Y, which we, females, do not have), they cannot have two copies for either of these two genes - they just have one copy of each. (This is also why the “X-linked” diseases such as haemophilia occur much more frequently in men than in women.)


by Daniela Kročianová

Year 2, Issue 3