Rods and Cones Photoreceptors
There are certain visual pigments, which are present in some specialized cells in the eye called, photoreceptors. Two kinds of photoreceptors are rods and cones. Rods are responsible for the absorption of light. Cones, on the other hand, are responsible for color vision. Defects in color vision can either be inherited or acquired. Inherited disease may be due to the absence of or incorrect visual pigments. However, acquired color vision defects may result due to eye disease, aging or side effects of certain medications.
The color vision deficiency falls into three major categories namely red-green color blindness, blue-yellow color blindness and a complete absence of color vision called, achromatopsia, which is uncommon.
How common is red-green color blindness?
Among the three categories, red-green color blindness is the most common eye vision disorder. This condition is more prevalent in male populations than females.
Causes of red-green color blindness:
Mutations in certain genes are responsible for color blindness. There are three kinds of cones (photo pigments), which are present on the retina, a light-sensitive tissue present on the backside of the eye. Specific genes are responsible for making the three photo pigments. The OPN1LW gene is responsible for making a pigment, which is very sensitive to light at the red end of the visible spectrum. Cones having this pigment are sometimes referred to as long-wavelength-sensitive or L cones.
The OPN1MW gene, on the other hand, is responsible for making a pigment, which is sensitive to yellow/green light and cones having this pigment is referred to as middle-wavelength-sensitive or M cones. Moreover, the OPN1SW gene is responsible for making a pigment that is sensitive to blue/violet light and cones having this pigment are generally called as the short-wavelength-sensitive or S cones. Defects in the OPN1LW and OPN1MW genes cause red-green color blindness.