For some time now, I’ve been wanting to write about red-green color blindness, a dramatic perceptual difference with an interesting genetic and evolutionary story. This first post will mostly be an introduction to the topic. If you are color blind: I always feel guilty when I speak of this as a deficiency, or when I emphasize how profound the differences seem to the rest of us. I hope it doesn’t bother you. I always wish I could pee standing up so… there.
Daylight vision in humans is mediated by the opsin proteins, which transmit signals that activate nerves when they are hit with light. Humans have three different opsins with different sensitivities to the colors of the spectrum — it is the different color sensitivities that allow us to see color. You can call these the “blue”, “green” and “red” opsins.
A normalized diagram of the sensitivities of opsins to different wavelengths of light.
In its severe form, red-green color blindness occurs when a man is missing the “green” or “red” opsin – these conditions are respectively known as deuteranopia (1% of all males) and protanopia (another 1% of males). They are fairly similar in effect: a total loss of ability to distinguish hues in the green to red range. There are many less severe forms of color blindness — 6% of males — but that’s a later post.
I say “males” because color blindness is almost always seen in men. This is because the “red” and “green” opsin genes are located on the X chromosome, which men have only one copy of. Women have two X chromosomes; even if one has inherited a deletion mutation, the other can serve as a back-up. For a woman to be color blind, both X’s would have to carry the same mutation, which is much less likely to occur. (e.g. 1% * 1% = 0.01%)
I’ll end this post by showing you what color blindness looks like. Vischeck is a service available online that simulates how images look to a color blind person. To a color blind individual the simulation and original images should look identical (or nearly so – computer monitors vary, so this cannot be perfect). If you’re curious about the algorithm, the program is based on this paper.
All colors in the red to green range — green, yellow, orange, red — are simulated here as yellow. As you can see, deuteranopia and protanopia are almost identical – the main difference is that red looks darker to the protanope (look closely at the picture of cars). Also interesting to note: the butterfly picture demonstrates how purple looks like blue to the color blind individual.
Credits: Opsin sensitivity diagram adapted from Wikipedia diagram, credit goes to User:Vanessaezekowitz and from the screenshot for Wavelength 1.3. Photos taken from flickr users Marshall Flickman, Teo, and Oneras under CC and CC-by-SA licenses.