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Normal color vision in humans

Normal color vision in humans is trichromatic, meaning it is the result of the presence of three different photopigments, each of which has a distinct spectral sensitivity, and the photopigments responsible for these sensitivities are carried by three different types of photoreceptor cells (Jagla, Jagle, Hayashi, Sharpe and Deeb, 2001, 23-32). The short wave length sensitive cells are responsible for sensing blue (B); the middle wavelength sensitive cells are responsible for sensing green (G); and the long wavelength sensitive cells are responsible for sensing red «). The photoreceptor cells carrying the three different photopigments are the cones. All three photopigments share the same conjugated chromophore, 11-cis-retinal, but differ in their opsin component, a covalently linked heptahelical transmembrane protein.

The genes which code for the G and R opsins are arranged on chromosome Xq28 in a head to tail tandem array (Jagla, Jagle, Hayashi, Sharpe and Deeb, 2001, 23-32). This arrangement has been visualized directly by in situ hybridization. A single R-photopigment gene is found on the 5' position of the array, and this is followed by one or more G-photopigment genes. An upstream locus control region (LCR) has been shown to exist upstream from the R-photopigment gene. This LCR has been shown to be responsible for cone photoreceptor-specific expression of all the genes present in the array.

The R- and G-photopigment genes possess a translational region of six exons and are highly homologous in their coding sequences (Jagla, Jagle, Hayashi, Sharpe and Deeb, 2001, 24). These coding sequences differ in only 15 amino acids. The high degree of homology predisposes these genes to unequal recombination, which leads to gene deletions and the formation of red-green hybrid genes which are responsible for most of the common red-green color vision deficiencies (Deeb and Kohl, 2003, 170-187).

The two most proximal gen...

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Normal color vision in humans. (1969, December 31). In LotsofEssays.com. Retrieved 07:41, March 29, 2024, from https://www.lotsofessays.com/viewpaper/1701943.html