Congenital color vision deficiencies are inherited conditions that a person is born with.
There are three types of cone cells in the retina that allow an individual to perceive three basic colors – blue, green and red. Most humans are born as what are called trichomats which means they can perceive all three of these colors. Among individuals with a color vision deficiency, however, one type of cone cell is either not functioning normally or missing. These people may be monochromats, in which case they can only perceive color in one dimension, or dichromats who can see mixtures of two colors. Although color vision deficiency can be caused by some illnesses or as a side effect of some medications, in the majority of cases it is an inherited condition that a person is born with.
The most common form of inherited color deficiency is the red–green deficiency, which is passed onto offspring via 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. Therefore, this type of colour vision deficiency is significantly more common among men than women. The blue–yellow vision deficiency is passed on through a non-sex chromosome and is equally common among men and women.
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. Aside from the effect monochromacy has on vision, people with this condition may demonstrate nystagmus, an involuntary, side-to-side movement of the eye. They may also have a sensitivity to light and a visual acuity of between 20/50 and 20/400.
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. Deuteranopia refers to an inability to see green light and in the case of 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 refers to a reduced sensitivity to green light and tritanomaly refers to a reduced sensitivity to blue light.