Colour vision deficiency is a problem that involves the retina, or more specifically, the cells within the retina. Located at the back of the eye, the retina is made up of two types of cells known as rods and cones. Rods are light-sensitive structures that provide night and low light vision. They are not associated with colour differentiation. That role is up to the cones. There are three types of cones and each is responsible for red, green or blue colour pigments. These so called "primary colours" allow us to see every other colour. If the cones that are associated with a colour are absent or not functioning properly, the affected person will have difficulty distinguishing certain colours.
The most common form of colour vision deficiency is deuteranopia, also known as "daltonism". In this case, the cones associated with the green photoreceptors are absent. As a result, colours such as green, orange, brown and light red are perceived as hues of grey and are difficult to tell apart. Protanopia, associated with the colour red, and tritanopia, associated with blue, are other forms of colour vision deficiency. They are however, quite uncommon.
Some people do indeed have only one cone type, while others have none at all. When there is a total absence of cones, everything is seen in black and white, a condition known as monochromatism (or achromatopsia).
Colour blindness, for the most part, is hereditary. It is inherited through the X chromosome. Men only have one X chromosome, while women have two. This means that for a woman to be colour blind, both her parents must be carriers and both must pass the defective gene on to her. If a woman only has one defective X chromosome, the problem will not manifest itself. It is therefore possible for a woman to be a "carrier" without having any symptoms.
Since men only have one X chromosome, which is automatically passed on from the mother, they are more likely to be affected as they only need one defective chromosome. This is why there is a much higher incidence of colour blindness in men than in women. Approximately 8% of Caucasian (white) males are affected, compared to 0.5% of females. This disorder is more common in the Caucasian population (white) than in Asian, African or Native populations.
Similar disorders may occur as a result of a reaction to medication, an eye disease or a damaged optical nerve.
People with colour blindness are often unaware of their disorder. It is generally noticed by others in their entourage. There are a variety of tests available to determine one's ability to differentiate colours (ex. pseudoisochromatic plates). Optometrists and ophthalmologists are trained to assess this type of problem.
There is no treatment for colour vision deficiency. People with colour blindness are forbidden to hold various positions (ex: airline pilot). Nevertheless, those affected can live perfectly normal lives by developing tricks to determine colours. For example, a person who has difficulty differentiating green and red on traffic lights can rely on their position instead (red on top, green at the bottom).