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Human with normal vision possesses 3 cones, which correspond to blue (S), green (M) and red (L). What about tetrachromacy, where people have 4 cones in their retinae? What is the fourth cone exactly, and what color does that other cone correspond to?

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According to Deeb (2005), there are two relatively common types of causes for tetrachromacy in humans:

  • The common Ser180Ala polymorphism, which results in two spectrally different red pigments in the retina. This mutation also plays an important role in variation in normal color vision. This polymorphism most likely resulted from gene conversion by the green‐pigment gene.
  • Another common variation are several red/green pigment chimeras with different spectral properties. The red and green‐pigment genes are arranged in a head‐to‐tail tandem array on the X‐chromosome with one red‐pigment gene followed by one or more green‐pigment genes. The high homology between these genes has predisposed the locus to relatively common recombination events that give rise to red/green hybrids and deletions of green‐pigment genes. Such events constitute the most common cause of red‐green color vision defects. Only the first two pigment genes of the red/green array are expressed in the retina and therefore contribute to the color vision phenotype. The severity of red‐green color vision defects is inversely proportional to the difference between the wavelengths of maximal absorption of the photopigments encoded by the first two genes of the array. Women who are heterozygous for red and green pigment genes that encode three spectrally distinct photopigments have the potential for enhanced color vision.

Reference
Smeeb, Clin Gen (2005); 67:(5): 369-3

Further reading
- Color vision across species

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