Color blindness is a term that is often used to describe color vision deficiency. However, true color blindness is actually a rare condition where no colour is seen at all. Colour vision deficiency on the other hand, refers to an ability to see and identify colours clearly.
Color vision deficiency can be categorized into two main types:
Red-green deficiency – Patients with this form of the condition are unable to identify certain shades of red or green. This is the most commonly inherited form of color vision deficiency. 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 their one X chromosome, whereas in females it needs to be present on both of the X chromosomes. Therefore, this form of colour deficiency is significantly more common in men than women.
Blue-yellow deficiency – This is a more rare type of color deficiency where the person finds it difficult to differentiate between blue and green. Yellow may be perceived as grey or purple. The blue–yellow deficiency is passed on through a non-sex chromosome and is equally common among men and women
Classification of color vision deficiency
Color vision deficiency is usually an inherited condition but it can also be caused by some illnesses or the use of some medications. The condition arises due to abnormality of the retina where there are three types of cone cells that allow an individual to perceive three basic colors – blue, green and red. In color vision deficiency, one type of cone cell is either not functioning normally or is missing. There are therefore three types of inherited color vision deficiency which include:
Also called total color blindness, this is an extremely rare condition where two or all three of the cone cells are missing and colour and light perception is one dimensional.
Dichromacy refers to a condition where one type of cone cell is dysfunctional or missing, meaning a certain portion of the light spectrum cannot be seen. One form of the condition is called protanopia which refers to an inability to see red light and another is deuteranopia, where people are unable to see green light. In tritanopia, people cannot see blue light.
In this condition, all three types of cone cells are functional but in one cone type, the perception of light is slightly inaccurate. Protanomaly describes a reduced sensitivity to red light, deuteranomaly a reduced sensitivity to green light, and tritanomaly a reduced sensitivity to blue light.
Reviewed by Sally Robertson, BSc