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How can colour blindness result from a missing or nonfunctioning type of cone?
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The RYB color model is being illustrated on a color wheel.
According to the trichromatic theory of colour vision, humans typically possess three types of cones in the retina, each sensitive to different wavelengths of light: short (blue), medium (green), and long (red). Colour perception relies on the brain interpreting the relative excitation of these three receptor types.123
Colour blindness occurs when one or more of these cone types are missing or nonfunctioning, disrupting the standard trichromatic system. The specific impact depends on which cone type is affected:43
- Dichromacy: When one type of cone is absent, an individual is left with only two functional cone types, limiting their ability to distinguish certain hues. For example, protanopia results from the absence of red cones, while deuteranopia occurs when green cones are missing. In both cases, individuals often struggle to distinguish between red and green. Tritanopia occurs when blue cones are absent, making it difficult to distinguish between blue and yellow.43
- Monochromacy: In rare cases where two or three cone types are nonfunctional, individuals may perceive only shades of gray.43
Additionally, reduced sensitivity can occur if cones are present but function abnormally, such as in protanomaly or deuteranomaly.4