Colour Vision Defects

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Colour Vision Defects

Maureen Neitz, University of Washington, Seattle, Washington, USA
Katherine Mancuso, University of Washington, Seattle, Washington, USA
Jay Neitz, University of Washington, Seattle, Washington, USA

Published online: September 2011

DOI: 10.1002/9780470015902.a0006000.pub2

Full Article on Wiley Online Library

Abstract
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Abstract

Normal colour vision refers to the form of trichromatic colour vision shared by most individuals. It is mediated by three types of retinal cone photoreceptors, short‐ (S), medium‐ (M) and long‐ (L) wavelength‐sensitive. Colour vision defects can either be inherited as the consequence of genetic defects that affect the function of one or more cone type, or they can be acquired through exposure to neurotoxins or as secondary effects of systemic or ocular diseases. Among individuals with colour vision deficiency, there is tremendous variation in the capacity for colour vision, ranging from no colour vision to nearly normal. Viral‐mediated gene therapy for ocular diseases has been an area of intense investigation over the last decade. Because genetic mutations underlying colour vision deficiencies have been identified, gene therapy may be a viable option for curing various forms of colour blindness in the future.

Key Concepts:

Normal colour vision requires three classes of cone photoreceptor in the retina: short‐, medium‐ and long‐wavelength sensitive.
The hallmark feature of colour vision defects is a reduction in the number of different colours that can be distinguished from each other.
Colour vision defects can either be inherited or they can be acquired secondary to disease or through exposure to certain drugs or neurotoxins.
Mutations and rearrangements in the genes encoding the short‐, medium‐ and long‐wavelength sensitive photopigments are responsible for inherited colour vision deficiencies.
Gene therapy has been successfully used to treat colour blindness in animal models and these experiments pave the way towards cures for colour blindness in humans.

Keywords: colour vision deficiency; cone photoreceptors; red–green colour vision; congenital colour vision defects; cone photopigment; gene therapy

Article Title:	Colour Vision Defects
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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 the 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 colour appearance of different wavelengths to a person with normal colour vision. (b) Two‐dimensional representation of L and M opsins. Balls represent amino acids. Grey 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.
	Figure 2. Comparison of dichromatic and normal colour vision. (a) The colours of the visible spectrum as they appear to a person with normal colour vision (left) were digitally altered (right) to illustrate the appearance of the same spectrum to a red–green colour blind dichromat. (b) Photograph of red and green peppers (left) digitally altered to illustrate the appearance of the same peppers to a red–green colour blind dichromat (right). There are two properties of colour: hue and brightness. A person with normal colour 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 colour.
	Figure 3. Recombination between X‐chromosome pigment gene arrays required to produce arrays observed in the present‐day population underlying normal, protan and deutan colour vision. (a) Intergenic recombination between ancestral two‐gene arrays that confer normal colour vision gives rise to one new array that confers normal colour vision and another that confers dichromacy (deuteranopia). (b) Intragenic recombination between two two‐gene arrays that confer normal colour vision produces two new arrays that both confer colour 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.