Any disorder of vision in which colours are confused. The most common type is red-green colour blindness. This is due to a recessive gene carried on the X chromosome (see sex linkage), and therefore men are more likely to show the defect although women may be carriers. It results in absence or malfunctioning of one or more of the three types of cone cell responsible for colour vision. In protanopia the individual lacks cones sensitive to red light; in deuteranopia cones sensitive to green light are absent. Tritanopia is a rare form of colour blindness in which the individual cannot distinguish between blue and green due to a lack of cones sensitive to blue light.