daltonism

n. The inability to distinguish between red and green usually due to a recessive genetic defect on the X chromosome which causes the absence of the forms of cones in the retina which make the red-green distinction. It occurs about 20 times as often in men as women (8% vs. 0.4%) as women have two X chromosomes while men have only one, and the probability that both X chromosomes will be defective is about one-twentieth the probability that only one will be defective. Also called red-green color blindness.