Globally, 1 in 12 men and 1 in 200 women have color vision deficiency or color blindness. Red-green color blindness is the most common form and is passed from mother to son via chromosome 23, known as the sex chromosome. There are many types of color blindness. Humans and most primates are trichromatic, meaning they have three light-sensitive proteins that help them distinguish between reds, blues, and greens. Some people with color blindness have defective trichromatic perception while others have dichromatic perception (two colors), monochromatic perception (one color), or none at all.
The human eye has a color spectrum of between 390 and 700 nanometers, which is a total of seven million colors. Within the fovea centralis, a small pit close to the optic nerve, are approximately six to seven million photoreceptors or cones of three types: S-cones, M-cones, and L-cones, with the letters standing for short, medium, and long-wavelength light. Wavelengths of reflected lights control the colors the eyes see. Any damage to or absence of the cones results in color blindness.
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