|MadSci Network: Medicine|
The back of the eye is covered in a thick layer of neural tissue called the retina. The inner layer of the retina is composed entirely of specialized photo-receptor cells which convert photons of light into nerve impulses. There are two sets of photo-receptor cells in the human eye: rods, which detect all visible light and are responsible for brightness, contrast, shape, and movement; and cones, which detect different wavelenghts of light, and are responsible for color vision. Cones normally come in three flavors, red, green, and blue, in ascending order of detected wavelengths. There are four major forms of color blindness. Total color blindness is the complete lack of cones in the retina, and is incredibly rare. At the opposite end, Anomalous Trichromatism is a minor defect in one set of cones, corresponding to a slight loss of one color. Monochromatism is also very rare, and involves the loss of two sets of cones, giving the sufferer modified black and white vision. The most common form of color blindness is Dichromatism, in which one set of cones is missing, but the other two function normally.
Dichromatism can be subdivided into three diseases: Protanopia is the lack of red cones; Deuteranopia is the lack of green cones; and Tritanopia is the lack of blue cones. While Protanopia and Tritanopia are both rare, Deuteranopia affects a large portion of the population. Deuteranopia is an X-chromosome linked disorder which afflicts men over women at a ratio of 20:1. The genetic linkage to Red-Green Dichromatism was originally clouded by the overlapping disorders of Protanopia and Deuteranopia. It wasn't until the two were distinguished medically that the underlying causes of common color blindness could be identified.
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