Color vision deficiency
The inability to differentiate certain shades of color is called color vision deficiency.
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Color vision deficiency
Color vision deficiency is more common in men than women due to its sex-linked inheritance. The defective gene is found on the X chromosome. Males have one X and one Y chromosome, while females have two X chromosomes.
Males have color vision deficiency if their X chromosome contains the defective gene. Females only have this condition if both of their X chromosomes include this gene. Therefore, color vision deficiency affects about 0.5 % of women and 8 % of men.
Color vision test
The retina contains receptors called cone and rod cells. Their impulses are transmitted to the bipolar cells which stimulate the ganglion cells. Axons in the ganglion cells make up the retinal nerve fiber layer. Rhodopsin is the light-sensitive pigment in rods consisting of retinal, a derivative of vitamin A, and opsin, a group of proteins. Rod cells cannot differentiate between colors as rhodopsin is stimulated by light of all wavelengths.
Their stimulation threshold is low: they even respond to a single photon. Therefore, they are also active when there is not enough light. There are three kinds of cone cells which are sensitive to red, green and blue light, respectively. Their light-sensitive pigment is called iodopsin, which differs from the rhodopsin in the group of proteins.
Their stimulation threshold is higher than that of rods, so they are not active when there is not enough light. This is why we see objects in shades of gray at dusk. Dim stars are best seen with our peripheral vision because then their image is not projected onto the macula lutea, but on the outside of the retina, which is rich in sensitive rods.
If any type of cone is missing or malfunctioning, color vision deficiency occurs, the most typical being red-green deficiency. If all three types are not functioning well, the patient sees in shades of gray; this is called color blindness.
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