clipped from: www.post-gazette.com A tetrachromat is a woman who can see four distinct ranges of color, instead of the three that most of us live with.
Each of the three standard color-detecting cones in the retina -- blue, green and red -- can pick up about 100 different gradations of color, Dr. Neitz estimated. But the brain can combine those variations exponentially, he said, so that the average person can distinguish about 1 million different hues.
A true tetrachromat has another type of cone in between the red and green -- somewhere in the orange range -- and its 100 shades theoretically would allow her to see 100 million different colors.
That's because the genes for the pigments in green and red cones lie on the X chromosome, and only women have two X chromosomes, creating the opportunity for one type of red cone to be activated on one X chromosome and the other type of red cone on the other one. In a few cases, women may have two distinct green cones on either X chromosome.