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ABSTRACT

  • Color vision has intrigued scientists for several hundred years. Red-green color vision defects are common and among the first recognized X-linked traits. The molecular genetics of the visual pigments mediating normal and defective color vision have been elucidated and provide the basis for an understanding of the genetic basis of normal and abnormal color vision.

  • The retina is a displaced part of the brain and includes four different photoreceptors: rods containing rhodopsin and cones containing photopigments sensitive to either blue (short-wave), green (middle-wave), or red (long-wave). Normal color vision is trichromatic and is subserved by these three cone pigments. Rhodopsin is used for dim-light vision, whereas the various cone photopigments mediate vision in bright light and color vision. The photopigments have characteristic absorption maxima with wide regions of overlap. The four human visual pigments are similar in their amino acid sequences and are members of the heptahelical transmembrane receptor family that includes olfactory receptors. The red and green pigments differ by at most 15 amino acids. Differences at three residues (180, 277, and 285) largely account for spectral differences between these pigments.

  • The autosomal genes for rhodopsin and the blue pigment are located at chromosome 3q21–24 and 7q31.3–q32, respectively. The red-green pigment gene complex maps to a subterminal site on the long arm of the X chromosome (Xq28) linked to the loci for adrenoleukodystrophy, glucose-6-phosphate deficiency, and hemophilia. The red-green gene arrays are composed of a single red pigment gene (six exons) and one or more green pigment genes (six exons) located downstream (3′) of the red gene. About 25 percent of male Caucasians have a single green pigment gene, whereas the rest have two, three, or more green pigment genes. Almost half of Japanese and African-American males only have a single green pigment gene. The high homology of the red and green opsin genes (including introns) predisposes to unequal crossover and accounts for the numerical polymorphism. Gene expression studies suggest that only the two most proximal among several pigment genes are expressed in the retina. The ratio of red to green pigment mRNAs in human retinas varies widely (1–10, with a mode of 4).

  • Illegitimate recombination between the red and green pigment genes causes deletions or the formation of hybrid genes and explains the genetic basis of the majority of color vision defects. The deletion of green pigment genes leaves a single red pigment gene that is characteristically associated with deuteranopia (G). Affected individuals are dichromatic because they completely lack functional green cones. The finding of severe trichromatic deuteranomaly (G′) in a few individuals with this genetic makeup remains unexplained.

  • Exon 5 (which includes two residues that account for two-thirds of the spectral difference between the red and green pigments) plays a major role in spectral tuning. The recombinational exchange of exon 5 produces hybrid pigments with large spectral shifts and corresponding effects on color vision.

  • 5′ Green–red 3′ hybrid genes with or without additional green genes usually are ...

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