The short answer to this question is simple: genetics.  As with most things, the answer is rarely that simple.  First of all, “color blind” is not a single thing.  There are several different forms of color-blindness, and within each type, there are several forms.

To understand color-blindness, you have to know a little about how we see.  There is a thin layer of tissue at the back of the eye with millions of photo receptors.  These photoreceptors have two shapes, rods and cones.  Rods, of which there are vastly more of than cones, are responsive to dim light, making them useful for night vision or seeing in dark rooms.  Cones are responsive to bright light.  While the rods are all the same, all seeing the same colors, there are three different types of cones.  These three types of cones allow us to see colors across the spectrum from those colors with long wavelengths, like red, medium, like green and short wavelengths like blue or violet.  Parenthetically, colors with longer wavelengths then visible red colors are “infrared” while colors on the other end of the scale are “ultraviolet”.  There is a piece of trivia that will impress (or bore) your friends.

Color-blindness is when something goes wrong with one of the cones and the person cannot see certain colors correctly.  The most common form of color-blindness is called “red-green color-blindness”, which is a loss of, or limited function of, the red cone.

This is the form that is way more common in men.  As we mentioned, it was all in the genetics.  The information for the red cone is in the X chromosome.  Men only have one X chromosome, so of the genetic code is messed up, there is no back up.  Women on the other hand, have 2 X chromosomes, so even if one has the code that leads to color-blindness, it is usually offset by the good gene.  In this case, as with all X-linked inheritance, the mother can be a carrier, and can pass that mutation to her children, but she, herself, will not be affected.

Oddly enough, women who have the gene that would lead to red-green color blindness, or other vision related mutation, have a slight chance of being tetra-chromatic.  What’s that?  Well, with 3 types of cones, humans are tri-chromatic.  Each cone seeing a vast amount of colors, in all of the possible combinations, we can see about 7 million different colors.  Since women have 2 X chromosomes, for each gene, one is generally not used.  For some women who have some form of mutated cone, it could lead to a 4th distinctive cone.  Adding a 4th cone and all of the colors that it could see to the 7 million colors most people could see, a tetra-chromatic woman can see upwards of almost 100 million colors.

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There are two other forms of color-blindness which are extremely rare: Blue-Yellow, which is a problem with the blue cone and complete color blindness where the person cannot distinguish any color except shades of grey, like a black and white television.  These are caused by genes which are carried equally by men and woman, so while extremely rare, neither of these are sex-linked.