This difference between men and women is due to the fact that genes involved in the perception of the colors green and red are located on the X chromosome. And yet, men possess only one X chromosome (XY), whereas women possess two X chromosomes (XX). If a woman is red-green color blind, it means that her two X chromosomes are defective, whereas if a man suffers from color-blindness only his X chromosome is defective.
Being color-blinded doesn’t mean that you are unable to see color. You just perceive color differently. Red-green color-blindness, which makes the patient unable to distinguish red and green, is the most common type of colorblindness. However, in rare cases, the inability to discriminate yellow and blue can be found. This second type of color-blindness is not sex-linked as the gene involved in the perception of blue resides in the chromosome 7, equally found in men and woman.
The perception of color is possible thank to cones located in the macula of the retina, which are photoreceptor cells. These cones transform the electromagnetic signal of light in an electric signal which is transmitted to the brain through the optic nerve. Colorblindness results from a deficiency in the development of cones, which are missing photoreceptors pigments.
Color-blindness is generally due to genetics, but sometimes it can result from brain, nerves or eyes damage. Red-green color-blindness can be detected at a very young age with the Ishihara color test.
This test was invented in 1917 by Shinobu Ishihara, professor at the University of Tokyo. This test consists in 38 plates. Each plate contains a number printed in dots of one color, surrounded by other dots of different colors. A color-blind person will have difficulty seeing the number, as for him the color of the number will merge with the color of the surrounding points.