While some people are colorblind, it appears other people are color-strange.
How color vision works and fails, down to the level of molecules, is illuminated in two research papers in the April 11 issue of Science magazine. Jeremy Nathans of Stanford University School of Medicine led a group that is the first to locate and map genes responsible for making colored pigments used by the eye in color vision.
The work confirmed the basic theory of color vision but added surprises, including existence of a group of people, called "anomalous trichromats," who see the world differently than normal people, not because of lack of color but because of the presence of a different pigment than the usual red, green and blue.
Eyes see when light strikes the array of rod- and cone-shaped cells at the back of the eye. The cone-shaped cells, about six million in each eye, are responsible for color vision. Each cone in normal people is specialized.
Each cone has one pigment dominating, and these react according to the amount of their own color coming into the eye. Low levels of blue stimulate the blue-receiver cones a little, while strong blues excite the cell and nerves linked to them.
The brain, receiving signals about the amount of light of each color coming into the eye, mixes them to produce the infinite range of colors humans see.
The genes that make red and green pigment are adjacent to one another on the X chromosome, the one that determines sex. Because males have only one X chromosome, if something goes wrong with the color pigment genes, there is no spare chromosome for backup, so colorblindness is a male trait.
Because red and green pigment genes are adjacent on the chromosome strand, they can sometimes overlap and disturb the functioning of either a red or a green pigment-producer.
More interesting is the possibility that mixing red and green genes will produce a hybrid whose pigment is neither red nor green, but is anomalous and reacts to a different range of light coming into the eye.
"What is particularly engaging about this kind of result . . . is the possibility that [it] might represent an example of ongoing further evolution of the visual system," MIT biologist David Botstein wrote.
Because it appears that the present system evolved from one that had only two colors, separating red and green to become a three-pigment system might allow hybrid genes to evolve into a fourth pigment "with a new spectrum that could extend the visual abilities of those who inherit it," Botstein wrote.