the bottom line is that men are much more likely to be color-blind. about 8% of human males are color-blind, as opposed to only about 0.4% of human females. but why is this so?

in our eyes we have photoreceptors called cones and rods. when light hits these photoreceptors, they send electrical signals to the brain, which is basically how we see. there is only one type of rod, but there are three types of cones. each type of cone reacts slightly differently to light of different frequencies. there is one that reacts strongly to red light, one to green light, and one to blue light. by combining the input from these three types of cones, the brain is able to see the full spectrum of colors which we can see.

each cone reacts to light by means of a protein which reacts to photons. the three proteins are very similar (as they do a very similar function), but are coded for by three separate genes. The three genes lie on two chromosomes. One (the "blue" one), sits on an autosomal chromosome (i.e. not X or Y). This means that both men and women have two copies of it. The two others (green and red), sit on the X chromosome. This means that women have two copies of it, but men have only one. On top of that, the two genes sit very close to one another on the X chromosome, which means they can easily recombine.

a faulty gene will mean that the protein that the gene would have coded for will not be made. The respective cone will be faulty, and the person will get no input from it. So s/he will have to deduce the colors of the world from only two cones. it has been shown (it is quite easy to do so, but nevertheless i will not here - if you wish, you may read more on the subject) that we need three separate (unrelated) cones. it doesn't really matter which three frequencies they respond to, as long as they are far enough from each other. anyway, two cones are not enough to distinuish all the colors.

a defective gene in a woman will not cause her to be color blind; a woman needs two defective genes so that there will be no gene to code for the protein, as a woman has two X chromosomes. men, however, have only one copy of each gene, so that any mutation in the gene means the man will be color-blind.

as the two cones in question are red and green, the most common color-blindness is the inability to distinguish between red and green.

it is interesting to note that

  1. not all animals have three different cones. many animals have only two, and many birds have four, which makes you wonder what shades they can see which we can't even dream of (birds can also see ultraviolet, but that is beside the point).
  2. new world monkeys (NWM)s have an interesting color vision. they have one autosomal gene and one gene on the X chromosome. but the gene on the X chromosome has some variability. there are two types of it (for sake of comfort, let us call them "green" and "red", and the autosomal gene "blue".) males have blue and either green or red, so see dichromatically (with two colors). females have blue, and two X chromosomes. on them, they can have two green genes, two red genes, or one red and one green. this means that some female NWMs can see trichromatically, but no male can!

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