Normal color vision in humans is trichromatic, being based on 3 classes of cone that are maximally sensitive to light at approximately 420 nm (blue cones; 613522), 530 nm (green cones; 300821), and 560 nm (red cones; 300822). ... Normal color vision in humans is trichromatic, being based on 3 classes of cone that are maximally sensitive to light at approximately 420 nm (blue cones; 613522), 530 nm (green cones; 300821), and 560 nm (red cones; 300822). Comparison by neural circuits of light absorption by the 3 classes of cone photoreceptors allows perception of red, yellow, green, and blue colors individually or in various combinations. Dichromatic color vision is severely defective color vision based on the use of only 2 types of photoreceptors, blue plus green (protanopia; see 303900) or blue plus red (deuteranopia). Anomalous trichromacy is trichromatic color vision based on a blue, green, and an anomalous red-like photoreceptor (protanomaly), or a blue, red, and an anomalous green-like photoreceptor (deuteranomaly). The color vision defect is generally mild but may in certain cases be severe. Common variation in red-green color vision exists among both normal and color-deficient individuals (review by Deeb, 2005).
Studies using reflection densitometry and retinal microbeam experiments showed that 2 different pigments mediate red and green sensitivity. These are located in the cones, each cone containing only 1 type of pigment (Waaler, 1968).
Simunovic et ... Studies using reflection densitometry and retinal microbeam experiments showed that 2 different pigments mediate red and green sensitivity. These are located in the cones, each cone containing only 1 type of pigment (Waaler, 1968). Simunovic et al. (2001) examined red-green color-deficient subjects, a small sample of monochromats, and age-matched color-normal control subjects to determine whether color vision deficiency confers a selective advantage under scotopic conditions. They found no evidence that red-green color deficiency or monochromatism confers a selective advantage under scotopic conditions, including dark adaptation, scotopic visual field sensitivity, or performance on a scotopic perceptual task.
Nathans et al. (1986) determined that whereas there is a single red pigment gene, green pigment genes vary in number among persons with normal color vision. The multiple green pigment genes are arranged in a head-to-tail tandem array. ... Nathans et al. (1986) determined that whereas there is a single red pigment gene, green pigment genes vary in number among persons with normal color vision. The multiple green pigment genes are arranged in a head-to-tail tandem array. The existence of multiple green pigment genes in tandem array may explain why deutan colorblindness is more frequent than protan colorblindness. Furthermore, nonhomologous pairing and unequal crossing-over can explain the development of colorblindness. Gene conversion may also be involved. The green pigment genes vary in restriction pattern. Although there are 15 amino acid differences between the MW (green) and LW (red) opsins, the greater part of the spectral shift in sensitivity is the result of substitutions at sites 180, 277, and 285, with 5 other sites having smaller effects. Site 180 (see 300822.0002) is polymorphic in both MW and LW opsin genes. The middlewave opsin is missing or defective in deuteranopia and the longwave opsin in protanopia. Using refined methods, Neitz and Neitz (1995) reexamined the numbers and ratios of genes in the Xq28 cluster in men with normal color vision. Results indicated that many men have more pigment genes on the X chromosome than had previously been suggested and that many have more than 1 longwave pigment gene. Deeb (2005) noted that the high homology between the red and green pigment genes has predisposed the locus to relatively common unequal recombination events that give rise to red/green hybrid genes and to deletion of the green pigment genes. Such events constitute the most common cause of red-green color vision defects. Only the first 2 pigment genes of the red/green array are expressed in the retina and therefore contribute to the color vision phenotype. The severity of the red-green color vision defects is inversely proportional to the difference between the wavelengths of maximal absorption of the photopigments encoded by the first 2 genes in the array. Winderickx et al. (1992) found that only a single green pigment gene is expressed in persons with normal color vision. They suggested that a locus control-like element (300824), already known to be located 3.8 kb upstream of the transcription initiation site of the red pigment gene (300822), allows transcription of only a single copy of the green pigment genes, probably the most proximal copy. This finding provided an explanation for the not infrequent presence of 5-prime green-red hybrid genes in individuals with normal color vision. Although such hybrid genes are usually associated with defective color vision, this may not occur when their position in the gene array does not allow expression in retinal cone cells. The defect of color vision in deuteranomaly (found in 5% of males of European descent) is associated with a 5-prime--green-red--3-prime visual pigment hybrid gene, which may also exist in males with normal color vision. To explain why males with a normal red, a normal green, and a green-red hybrid gene may have either normal or deutan color vision, Winderickx et al. (1992) and Yamaguchi et al. (1997) hypothesized that only the first 2 genes are expressed and deuteranomaly results only if the green-red hybrid gene occupies the second position and is expressed preferentially over normal green-pigment genes occupying more distal positions. Hayashi et al. (1999) used long-range PCR amplification and studied 10 deutan males (8 deuteranomalous and 2 deuteranopic) with 3 visual pigment genes (red, green, and green-red hybrid) to investigate whether position of the hybrid gene in the array determined gene expression. The green-red hybrid gene was always at the second position (and the first position was always occupied by the red gene) in men with the deutan defect. Conversely, in 2 men with red, green, and green-red hybrid genes and normal color vision, the hybrid gene occupied the third position. When pigment gene mRNA expression was assessed in postmortem retinas of 3 men with the red, green, and green-red genotype, the green-red hybrid gene was expressed only when located in the second position. Since only the first 2 genes are expressed, the retinas of deuteranomals are presumably composed of cones containing red-sensitive pigment and cones containing a red-like--sensitive pigment. The findings of Hayashi et al. (1999) were consistent with the presence of a locus control region (LCR) at the 5-prime end of the X-linked visual pigment gene. This LCR was postulated to form a stable transcriptionally active complex in a stochastic manner with either the red-gene promoter to form red-sensitive pigment, or with the green-gene promoter to form green-sensitive pigment. The LCR is presumably too far removed from the third gene to affect its expression. Another explanation would be that distal gene expression is silenced by elements in the 3-prime-flanking region of the locus. Although the data came from individuals with 3 pigment genes, these findings presumably apply also to lack of expression of visual pigment genes in the fourth or even more distal positions. Jagla et al. (2002) investigated the genotypic variation in 50 red-green color vision-deficient males (27 deuteranopes and 23 protanopes) of middle European ancestry who possessed multiple genes in the X-linked photopigment gene array. Spectral sensitivities of the encoded pigments were inferred from published in vitro and in vivo data, and color vision phenotype was assessed by standard anomaloscopy. Most genotypes included hybrid genes whose sequence and position and whose encoded pigment correlated exactly with the phenotype. However, a few of the protanopes had gene arrays consistent with protanomaly rather than protanopia, since spectrally different pigments may be encoded by their arrays. Two of the deuteranopes had only R- and G-photopigment genes, without any detectable G/R-hybrid genes or identified mutations. About half of the protanopes possessed an upstream R/G-hybrid gene with different exon 2 coding sequences than their downstream G-pigment gene(s), which is inconsistent with published data implying that a single amino acid substitution in exon 2 can confer red-green color discrimination capacity on multigene protans by altering the optical density of the cones. Up to the time of the report by Winderickx et al. (1992), all red-green color vision defects had been associated with gross rearrangements within the red/green opsin gene array on Xq28. In a male with severe deuteranomaly without a rearrangement of the red/green pigment genes, Winderickx et al. (1992) found that substitution of a highly conserved cysteine by arginine at position 203 (C203R; 300821.0001) in the green pigment opsins accounted for his defect in color vision. Surprisingly, this mutation was found to be fairly common (2%) in the population but apparently was not always expressed. Ueyama et al. (2003) studied 247 Japanese males with congenital deutan color vision deficiency and found that 37 subjects (15%) had a normal genotype of a single red gene followed by 1 or more green genes. Two of the patients had previously been found to have a missense mutation in 1 or more green pigment genes (300821.0003 and 300821.0004) (Ueyama et al., 2002), but the other 35 had no mutations in either the exons or their flanking introns. However, 32 of the 35 subjects, including all 8 subjects with pigment color defect (a special category of deuteranomaly), had a -71A-C transversion (300821.0005) in the promoter of a green pigment gene at the second position in the red/green visual pigment gene array. Although the -71C substitution was also present in color-normal Japanese males at a frequency of 24.3%, it was never at the second position but always found farther downstream. The substitution was found in 19.4% of Chinese males and 7.7% of Thai males, but rarely in Caucasians or African Americans. These results suggested that the -71A-C substitution is closely associated with deutan color vision deficiency. In Japanese and presumably other Asian populations, farther downstream genes with -71C comprise a reservoir of the visual pigment genes that cause deutan color vision deficiency by unequal crossing-over between the intergenic regions. - Reviews Nathans (1987) reviewed the molecular biology of colorblindness. Deeb (2005) reviewed the molecular basis of variation in human color vision.
The frequency of red-green color vision defects among populations of northern European origin is around 8% of males and 0.5% of females, as determined by anomaloscopy in many studies. The frequency is lower in non-European populations (review by ... The frequency of red-green color vision defects among populations of northern European origin is around 8% of males and 0.5% of females, as determined by anomaloscopy in many studies. The frequency is lower in non-European populations (review by Deeb, 2005). Drummond-Borg et al. (1989) found abnormalities of color vision pigment genes in 15.7% of Caucasian men, a higher frequency than is shown by color vision testing. This may indicate that some color vision gene arrays associated with hybrid genes mediate normal color vision.