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Color Vision Defects

  1. Maureen Neitz,
  2. Jay Neitz

Published Online: 27 JAN 2006

DOI: 10.1038/npg.els.0006000

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How to Cite

Neitz, M. and Neitz, J. 2006. Color Vision Defects. eLS. .

Author Information

  1. Medical College of Wisconsin, Milwaukee, Wisconsin, USA

Publication History

  1. Published Online: 27 JAN 2006

This is not the most recent version of the article. View current version (15 SEP 2011)

Introduction

  1. Top of page
  2. Introduction
  3. Inherited Color Blindness
  4. Acquired Color Blindness
  5. See also
  6. References
  7. Further Reading
  8. Web Links

The biological basis of color vision can be divided into two stages: the first is the light-sensitive cone photoreceptor cells in the retina and the second is the neural components that process information about wavelength gathered by the photoreceptors. Humans with normal color vision have three populations of cone photoreceptors that are classed according to their relative spectral sensitivities as short-, medium- and long-wavelength sensitive, and abbreviated S, M and L respectively. Red–green color vision is mediated by neural circuitry that compares the quantal catches of L and M cones; blue–yellow color vision is mediated by circuitry that compares the summed quantal catches of L and M cones with the quantal catch of S cones. People with normal color vision can distinguish more than a million colors, but people who are color blind see many fewer colors. Color blindness can be inherited or acquired. Inherited color blindness affects the first stage of color vision and is commonly caused by rearrangements, deletions and mutations of genes that encode the light-absorbing photopigment molecules in cones. Acquired color blindness arises through insult to the visual system, for example through drug or chemical toxicity, disease or trauma. See also Visual Pigment Genes: Evolution

Inherited Color Blindness

  1. Top of page
  2. Introduction
  3. Inherited Color Blindness
  4. Acquired Color Blindness
  5. See also
  6. References
  7. Further Reading
  8. Web Links

Inherited color blindness can be categorized according to the number of functional cone types present in the retina. Autosomal recessive achromatopsia is a rare disorder associated with the absence of functional cones (complete achromatopsia) or with residual cone function (incomplete achromatopsia). Defects in the alpha and beta subunits of the cone photoreceptor-specific cyclic guanosine monophosphate (GMP) gated ion channel are known causes (Sundin et al., 2000; Wissinger et al., 2001). All three cone classes require the function of this ion channel to generate an electrical signal in response to light. It has been suggested that mutations causing complete achromatopsia abolish ion channel function, whereas incomplete achromatopsia is associated with mutations that allow residual channel function.

Monochromacy, dichromacy and anomalous trichromacy are forms of color blindness characterized by the presence of one, two or three functional types of cone, respectively. All are caused by mutations that lead to an altered complement of functional cone photopigments expressed in the retina. Photopigments are light-absorbing molecules that determine the cone spectral sensitivities, and like cones are classed as S, M and L. Photopigments have two components: a transmembrane protein, termed opsin; and the chromophore, 11-cis-retinal. The chromophore is covalently attached to the opsin and is nestled in a hydrophobic pocket formed by opsin's transmembrane domain. Human L and M opsins share over 98% amino acid sequence identity and each is about 43% identical to S opsin (Nathans et al., 1986b). Photopigment absorption spectra are tuned by the amino acid sequence differences in the opsins (Neitz et al., 1991; Asenjo et al., 1994). Two amino acid substitutions together produce a large spectral shift that separates the pigments into L and M classes (Figure 1). Small shifts in spectrum resulting from normal amino acid polymorphisms create spectral variants of L and M pigments (Figure 1). Normal sequence variation in the S pigment has not been observed.

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Figure 1. Tuning of cone photopigment absorption spectra. (a) Absorption curves for S (blue curve), M-class (family of green curves) and L-class (family of red curves) pigments. Wavelength of peak absorption is 415 nm for S pigment, near 530 nm for M-class pigments and near 560 nm for L-class pigments. The rectangular bar below the x axis indicates the color appearance of different wavelengths to a person with normal color vision. (b) Two-dimensional representation of L and M opsins. Balls represent amino acids. Gray balls are invariant amino acid positions among normal L and M opsins. The black ball is the residue to which the chromophore is attached. Red balls are the two amino acid positions that produce the spectral difference between M- and L-class pigments. Yellow balls are positions that produce small spectral shifts and produce subtypes of M and L pigment. Blue balls are variant positions with no influence on the spectrum.

Tritan color vision deficiency or blue–yellow color blindness is caused by mutations in the S cone pigment gene (opsin 1 (cone pigments), short-wave-sensitive (color blindness, tritan) (OPN1SW)) on chromosome 7 at 7q31.3–q32, and it affects less than 1 in 10000 people (Nathans et al., 1992). The dichromatic form, tritanopia, is autosomal dominant with incomplete penetrance. Three amino acid substitutions in the transmembrane domain of S opsin are known causes. The existence of an anomalous trichromatic form and its possible cause is uncertain.

Red–green color blindness is extremely common and is caused by the absence of normal M or L cone function. The incidence varies with ethnicity, affecting 8% of Caucasian men, 4% of Japanese men and 3% of African men. Only about 1 in 230 Caucasian females is affected, but 15% are heterozygous carriers. The genes opsin 1 (cone pigments), long-wave-sensitive (color blindness, protan) (OPN1LW) and opsin 1 (cone pigments), medium-wave-sensitive (color blindness, deutan) (OPN1MW), encoding respectively the L and M opsins, lie in a head-to-tail tandem array on the distal tip of the long arm of the X chromosome at Xq28. Rearrangements, deletions and mutations within the array are the main causes of red–green color blindness. The X-chromosome location of the L and M genes accounts for the dramatic gender difference in the incidence of color blindness. Females have two X chromosomes, males have one. If a male inherits an X chromosome that confers expression of only one functional class of pigment, he will be color blind. A female will be affected only if both of her X chromosomes together confer expression of a single functional type of pigment.

Protan color vision defects are characterized by the absence of normal L cone function, and affect about 2% of Caucasian men: 1% are dichromats (protanopes) and 1% are anomalous trichromats (protanomalous). Deutan color vision defects are characterized by the absence of normal M cone function, and affect about 6% of Caucasian men: 1% are dichromats (deuteranopes) and 5% are anomalous trichromats (deuteranomalous). Figure 2 illustrates the difference between dichromatic and normal color vision. People with normal color vision see black, white, gray and four unique color categories, red, green, yellow and blue, which occur in combinations to produce thousands of intermediates, but for the dichromat, all colors appear as mixtures of just two hues (in this illustration, blue and yellow) with black, white and gray. The region of the spectrum between blue and yellow appears white or gray to the dichromat.

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Figure 2. Comparison of dichromatic and normal color vision. (a) The colors of the visible spectrum as they appear to a person with normal color vision (left) were digitally altered (right) to illustrate the appearance of the same spectrum to a red–green color blind dichromat. (b) Photograph of red and green peppers (left) digitally altered to illustrate the appearance of the same peppers to a red–green color blind dichromat. There are two properties of color: hue and brightness. A person with normal color vision can detect the difference in hue between bell peppers that do not differ significantly in brightness. A dichromat cannot detect the difference in hue, and the peppers appear to be all the same color.

Normal color vision requires at least one L, one M and one S pigment gene. The tandemly repeated L and M genes are prone to unequal homologous recombination and this has produced the diversity seen in the modern human population in the gene sequences, the number and arrangement of genes in the array and in color vision phenotype (Nathans et al., 1986a; Neitz et al., 1996). Both intragenic and intergenic recombination will produce new arrays with a different number of genes from the parental arrays (Figure 3). The ancestor to modern humans is believed to have had an array with one L and one M pigment gene. Unequal recombination between two such arrays will produce one array with three genes, and another with one gene (Figures 3a, 3b). A male who inherits an array with one gene is an obligate dichromat, and will be a protanope if the gene encodes an M-class pigment or a deuteranope if it encodes an L-class pigment.

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Figure 3. Recombination between X-chromosome pigment gene arrays required to produce arrays observed in the present-day population underlying normal, protan and deutan color vision. (a) Intergenic recombination between ancestral two-gene arrays that confer normal color vision gives rise to one new array that confers normal color vision and another that confers dichromacy (deuteranopia). (b) Intragenic recombination between two two-gene arrays that confer normal color vision produces two new arrays that both confer color blindness. (c) Intragenic crossover needed to produce protanomalous arrays. The parental three-gene array must be produced by crossover between two ancestral two-gene arrays, and this added step probably accounts for the lower frequency of protanomaly in the population. (d) To delete the M gene from a deutan array requires a crossover between a deutan array with an M gene and another array.

Chimeric genes in which parental L and M pigment gene sequences are intermixed arise from intragenic recombination (Figures 3b, 3c). Whether the chimeric gene will encode an M-class or an L-class pigment is determined by which parental gene contributes the sequences encoding amino acid positions 277 and 285. The most common array structure in deuteranomalous men is an L gene followed by a chimeric gene that encodes an L-class pigment, followed by an M gene (Figure 3b, 3c). The M genes in deutan arrays are not functionally expressed. Whether the chimeric and parental L genes encode spectrally identical L-class pigments depends on whether the pigments differ at spectral tuning sites. If the encoded L-class pigments have identical spectral properties, then a male with this array will be a deuteranope (dichromat), but if they differ in spectral properties he will be deuteranomalous (trichromat). Among deuteranomalous men, there is variation in the extent of loss of color vision. Generally, the more similar the L-class pigments are in spectral sensitivity, the poorer the person's color vision, and conversely the more dissimilar the L-class pigments, the better the person's color vision (Neitz et al., 1996).

Crossovers that produce arrays containing one or more genes encoding M-class pigments, but lacking genes for L pigments, give rise to protan defects (Figure 3b, 3c). A male inheriting an array with more than one gene encoding an M-class pigment will be protanomalous if the encoded pigments have different spectral properties, but he will be a protanope if they have identical spectral properties.

Although the genotype of the L/M gene array is a strong predictor of color vision phenotype, it is not 100% accurate. For example, some individuals would be predicted to be anomalous trichromats from genotype, but behave as dichromats. Rarely, males with red–green color blindness have an X-chromosome visual pigment gene array that is grossly indistinguishable from an array underlying normal color vision. In some cases, missense mutations that render either the L or M opsin nonfunctional have been identified; the most common mutation is substitution of arginine for cysteine at amino acid position 203 of the opsin.

Blue cone monochromacy is a rare inherited color vision defect characterized by the lack of functional L and M cones (Nathans et al., 1989). There are two known causes. First, some affected individuals have a single X-chromosome visual pigment gene that has a deleterious mutation. Second, about 40% of genetically characterized blue cone monochromats have a deletion of an enhancer that lies upstream of the X-chromosome visual pigment gene array and that is required for expression of the L and M genes (Nathans et al., 1993).

Acquired Color Blindness

  1. Top of page
  2. Introduction
  3. Inherited Color Blindness
  4. Acquired Color Blindness
  5. See also
  6. References
  7. Further Reading
  8. Web Links

Acquired color blindness generally affects all three cone classes, although not necessarily equally. It can be caused by toxicity, for example by exposure to ethambutol (used to treat tuberculosis), to drugs for treating hypertension and to solvents used in the plastics industry. Color vision loss is associated with systemic diseases such as multiple sclerosis and diabetes, and with ocular diseases such as glaucoma and optic neuropathy.

References

  1. Top of page
  2. Introduction
  3. Inherited Color Blindness
  4. Acquired Color Blindness
  5. See also
  6. References
  7. Further Reading
  8. Web Links

Further Reading

  1. Top of page
  2. Introduction
  3. Inherited Color Blindness
  4. Acquired Color Blindness
  5. See also
  6. References
  7. Further Reading
  8. Web Links
  • Birch J (1993) Diagnosis of Defective Colour Vision New York, NY: Oxford University Press.
  • Nathans J (1989) The genes for colour vision. Scientific American 260: 2449.
  • Nathans J, Merbs SL, Sung C, Weitz CJ and Wang Y (1992) Molecular genetics of human visual pigments. Annual Review of Genetics 26: 403424.
  • Neitz M and Neitz J (1998) Molecular genetics and the biological basis of colour vision. In: Backhaus WGK, Reinhold K and Werner JS (eds.) Color Vision: Perspectives from Different Disciplines, pp. 101119. New York, NY: Walter de Gruyter.
  • Neitz M and Neitz J (2000) Molecular genetics of colour vision and colour vision defects. Archives of Ophthalmology 118: 691700.
  • Neitz J, Carroll J and Neitz M (2001) Colour vision: almost reason enough for having eyes. Optics and Photonics News 12: 2633.
  • Piantanida T (1988) The molecular genetics of colour vision and colour blindness. Trends in Genetics 4: 319323.
  • Sharpe LT, Stockman A, Jägle H and Nathans J (1999) Opsin genes, cone photopigments, colour vision, and colour blindness. In: Gegenfurtner KR and Sharpe LT (eds.) Colour Vision: From Genes to Perception, pp. 352. New York, NY: Cambridge University Press.

Web Links

  1. Top of page
  2. Introduction
  3. Inherited Color Blindness
  4. Acquired Color Blindness
  5. See also
  6. References
  7. Further Reading
  8. Web Links