Normal color vision in humans
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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 si
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ne encoding the red-sensitive photopigment and/or in the region up to 17.8 kb upstream of the red gene (Ayyagari et al, 2000, 75-82). This is the area which contains the LCR and other regulatory sequences. These same nine families showed a range of deletions from the loss of just one single exon to the loss of the entire red gene when the red pigment gene was examined. In one family, there was complete loss of the green pigment gene. No association was found between genotype and phenotype in these families.
Michaelides et al (2004) looked at three British families with BCM to determine the molecular basis of the disease. Affected and unaffected family members were tested by clinical examination, electrophysiological and psychophysical tests. Blood samples were taken for DNA extraction (2). The coding regions of the long wavelength- «) and middle wavelength-sensitive (G) cone opsin genes and the upstream LCR were amplified by polymerase chain reaction, and fragments analyzed for mutations by direct sequencing.
The study confirmed the protan-like D-15 arrangements in patients with BCM, and demonstrated that the Mollin-Reffin (MR) Minimal test is a useful color discrimination test for diagnosing BCM (Michaelides et a
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Some common words found in the essay are:
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Approximate Word count = 2099
Approximate Pages = 8 (250 words per page)
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