Total color blindness is extremely rare. The most common type of color blindness is the inability to distinguish between red and green. Light enters the eye through the cornea, lens, eye fluid, and hits the retina, a meshwork of tightly packed nerve cells.
There are three types of receptors packed near the center of the retina. The light fires the nerve impulses and the electrical signals travel to the brain via the optic nerve. The center of the retina is lined with nerve cell receptors called cones that respond to red, green, and blue light. Red-green color blindness is due to a lack of those red receptors.
Color blind people learn by experience to compensate for their defect by associating certain colors with changes in the brightness of the light. Therefore, many colorblind people don’t know they have the defect. That’s why we take those ink blot tests at the drivers license bureau.
Colorblindness is hereditary and is caused by a recessive gene on the X chromosome. Only one healthy or dominant gene is needed for correct color vision. Since boys get a Y chromosome from their father and an X chromosome from their mother, boys who are colorblind must receive it from their mother. About 8% of men are colorblind, and about one-half of one percent of women are colorblind. These men cannot pass on their colorblind condition to their sons, since they pass on only a Y chromosome, not an X, to their son. Because females have two X chromosomes, if one is deficient or recessive, the other one is likely to be dominant and makes up for it. Again, males have an X and a Y chromosome, so if a boy is colorblind, he got it from his mother, who gives him only X’s. He can’t get colorblindness from his colorblind father, because he gets a Y chromosome from his father.
In order for a girl to be colorblind, she must receive a recessive X from the mother and a recessive X from the father. That rarely happens, so colorblindness is sixteen times more prevalent in boys compared to girls.
Colorblindness is not contagious. It is a lifelong condition, and there is no way to prevent it and no way to treat it. It is just something people learn to live with.