Color blindness is a term that is often used to refer to color vision deficiency. True color blindness is actually just one form of colour vision deficiency and is an extremely rare condition. Colour vision deficiencies are usually inherited disorders, although they may also occur as a side effect of some medications or as a result of certain illnesses.
The different forms of colour vision deficiency include:
Total color blindness
Also called monochromacy, this condition refers to a complete inability to differentiate colors. This is a very rare condition and usually manifests during childhood. In the healthy retina, three types of cone cells allow an individual to perceive three basic colors – blue, green and red. In total color blindness, two or all three of the cone cells are missing, meaning colour and light perception is one dimensional.
Red–green color blindness
Patients with this condition are unable to identify and differentiate certain shades of red or green. This is the most common form of inherited color vision deficiency. Protanopia refers to an inability to see red light while people with deuteranopia are unable to see green light. Red-green deficiency is inherited on the X chromosome. For males to have the disorder, a faulty gene for red-green vision only needs to be present on his one X chromosome whereas in females it needs to be present on both the X chromosomes. Therefore, this type of colour vision deficiency is significantly more common in men than women.
Blue-yellow color blindness
The blue–yellow vision deficiency is passed on through a non-sex chromosome and is equally common among men and women. The condition is also called tritanopia and the gene mutation responsible is thought to lie on chromosome 7.