Color Vision and Color Blindness
Humans, like any organism, are sensory dependent. They rely on information fed to them through several sources, including the eyes. The eyes of a human are complex instruments that have historically held a high fascination for mankind. For instance, every romantic song ever written has mentioned the female ocular at some point during the lyricist's rantings. Beyond this type of fascination, however, human eyes are at the forefront of many medical and genetical investigations, including those which seek to discover the causes of color blindness.
Colorblindness is a trait that reduces the ability of the human eye to perceive color. This disability is due to genetic failings resulting in poor expression of certain pigments. Certain of these pigments are encoded on the X gene, show high levels of conservation, are extraordinarily homologous, and suffer fairly frequent recombination. The other pigments involved with color vision are located on chromosome 7. In the following paragraphs, the molecular failings of this disease will be examined.
In normal eyes, color is detected by three types of photoreceptor cells: red, green, and blue (called so because they contain pigments that absorb the particular colors of nomenclature). The red and green cells are located throughout the fovea (a specific part of the retina) while the blue cells are located only along the periphery. Along the periphery, the red, green, and blue cells are aligned in a specific pattern that produces the type of color vision that most people experience. However, there can be problems, and most causes of color blindness involve pigment mutations in these photoreceptor cells.
All of the pigments contain a chiomafore, an integral membrane protein that is linked to a G-protein such that light of a very specific wavelength hitting the cell triggers it to become hyperpolarized. This polarization is done by photoisomeration and triggers a neural cascade leading to the perception of color depending on which pigment is stimulated. If light is created such that it only stimulates one type of photoreceptor cell (despite possibly consisting of multiple wavelengths), then only one color will result. However, if light of only one wavelength is shone into the eyes, it is possible that multiple types of photoreceptor cells will be stimulated due to overlapping ranges of absorption. This creates a large variety of types and degrees of color blindness, as any variation in number of each type of pigment, range of the photoreceptor cells, or positioning of the photoreceptor cells will alter the color(s) perceived by the eye. As shall be discussed in the oncoming paragraphs, there are different reasons and probabilities for each.
The primary cause of color blindness involves the "red" and "green" genes on the X chromosome. The red gene is the LWS, or long-wave sensitive pigment gene, while the green is the MWS, or mid-wave sensitive pigment gene. These alleles lie back to back on the X chromosome, with one LWS being followed by a variable amount of MWS. This has two significant implications. First, their position on the X chromosome means that, although recessive, males exhibit the phenotype resultant from a mutation more so than do females. And second, their back-to-back positioning precipitates a greater rate of recombination and unequal crossing over due to their great homology (96% of the LWS and MWS sequences are equivalent). In other words, the specific positioning present on the X chromosome allows the LWS and MWS to undergo frequent recombination that is easily expressed in males.
Color blindness can be tested for in two ways. An anomaloscope can be used. This instrument displays a yellow light to one eye and a mixture of red and green to the other. The subject is to adjust the levels of red and green light until the color of that object matches the color of the yellow object. The particular mix of red and green is diagnostic for many different forms of colorblindness. Secondly, Ishihara plates can be used. These plates feature discs covered in colored dots. Some dots of the same color are arranged into a pattern, often a number. The ability to perceive this pattern indicates that the subject can see that color.
These tests, however, only determine the presence or absence of color blindness; they do not attempt to treat it. At current, there is no treatment for color blindness despite the fact that it is a fairly widespread disability (up to 10% of the individuals in some populations). Although the affects are not severe and do not greatly hinder quality of life, more research obviously needs to be done in this area, although at current no new insights appear obvious.