Answer: Color vision deficiency is an inability to distinguish certain sets of colors, while colorblindness is the complete inability to perceive color.
Color vision deficiency cause
Our ability to see colors depends largely on a population of neuronal cells in the retina of the eye called cone-type photoreceptors. These are neurons that are able to change their electrical properties based on the presence of photons of light. The specific light sensitive protein is called photopsin.
There are three different types of cone-type photoreceptor cells. Each of these three types has a specific wavelength of light that they are most sensitive to.
S cones are most sensitive to blue light. The S stands for “short” since blue light is a shorter wavelength (around 440 nanometers) compared to other frequencies of light.
M cones respond most to green light. M stands for “medium”, and green is the color detected by wavelengths of light with a frequency around 500 nanometers.
L cones are maximally sensitive to red light, which is the color perceived by 600 nanometer photons.
For most people, activation of these three cones in different amounts allows us to see the colors of the rainbow. When the three cone types work together to produce color vision, it is called trichromatism.
Genetics of color vision deficiency (CVD)
However, in people with color vision deficiency (CVD), one or more of the cone photoreceptors are not functioning. There are many reasons why their cone cells do not work properly; most of these reasons are carried in the genes. Color vision deficiency is one of the most common genetic conditions. Many of the genetic causes of CVD are recessive and are linked to the X-chromosome. Because of this, it has a much higher prevalence among men compared to women. Some estimates put the prevalence close to 8% for men compared to 1% for women.
There are mainly three genes that are responsible for coding the proteins needed for color vision, called OPN1LW, OPN1MW, and OPN1SW. Each of these genes are responsible for coding a different version of the photopsin protein. Of the genetic causes of color vision deficiency, one is called deuteranopia, which is a defect in the OPN1MW gene. This gene codes for proteins found in M cones. Possible, the genes are simply deleted, but it is also possible that mutations in the genes cause the protein products to be non functional. In this case, a person may be diagnosed with deuteranomaly. When these genes make the incorrect protein product, a person has difficulty identifying red from green.
When the L cones are not functioning properly, such as when there is a mutation in the OPN1LW gene, the result is protanopia. People with protanopia have a difficult time distinguishing reds, greens, and yellows.
A mutation in the OPN1SW gene may cause defective S cones. These people may be diagnosed with tritanopia. They have a difficulty in distinguishing blue from yellow.
Color blindness
True colorblindness, on the other hand, is a complete inability to distinguish colors. It is also called achromatopsia. People who are colorblind see the world in grayscale.
People with colorblindness often exhibit an aversion to bright lights, a symptom called photophobia. Photophobia is a misnomer, however, since they exhibit only a physical aversion to the light rather than any psychological fear, which accompanies true phobias.
They also have very poor acuity, since the majority of the color sensing photoreceptor cells are located in the fovea of the retina, which is the area that is most responsible for high acuity vision.
Another common symptom that appears in these people is nystagmus, an uncontrolled an unintentional movement of the eyes. A nystagmus can be detected early in infancy.
Genetics of colorblindness (achromatopsia)
Achromatopsia is typically genetic, passed on in an autosomal recessive fashion. The genes involved in cone photoreceptor cells may have mutations which prevent them from functioning the way they are supposed to. These mutations can also lead to progressive degenerative loss of the cells. It is estimated that one out of 30,000 people have some degree of true colorblindness.
The majority of cases of achromatopsia result from mutations in the CNGA3 or CNGB3 genes. These genes code for a transmembrane protein found in cone photoreceptor cells called cyclic nucleotide gated channels.
These genes are distributed unevenly across the population. For example, on Pingelap Atoll in Micronesia, as many as 10% of the population have some form of achromatopsia. Dr. Olive Sacks described the medical mystery surrounding these people in his book, The Island of the Colorblind.
