COLOUR BLINDNESS
Created | Updated Mar 19, 2002
C L M Abel
2nd Year
Bsc. (Hons) Biological Sciences associated Environmental Sciences
Colour Blindness
Colour blindness is a disorder in which the individuals affected have a partial or total inability to detect certain wavelengths of the visual spectrum. It was first described by John Dalton, the famous chemist, in 1798, who described his own defective colour vision. Colour blindness is also known as Daltonism for this reason. In 1881 Lord Rayleigh placed the testing of colour discrimination was placed on a scientific basis. It was known that a mixture of red and green gave yellow, but Rayleigh showed that the particular mixture of a specific red and green, that a person made to match a standard yellow could be used as a test for colour discrimination. (Henthorne 1999). The total inability to see colour is rare and the disorder should more accurately be known as colour vision deficiency (Henthorne 1999), or colour defective vision (RACO 1999).
Colour Blindness affects 8% of white males and 4% of black males, and 1% of white females and 0.8% of black females in the United States (Gould and Keeton 1996) and 12 percent of European males and 0.5 percent of European females (Henthorne 1999). In Australia about 8% of all males and 0.5% of females are affected to a varying degree. In the study by Bell and Haldene on The Linkage between the Genes for Colour-blindness and Haemophilia in Man, in 1936 it was written that over 2% of human males are colour blind.
The cause of the disproportionately high numbers of males affected compared to females is that colour blindness is a X chromosome sex linked recessive disorder. This means that the loci for the most frequent types of colour blindness are on the X chromosome. The normal ability to see colours depends on several genes, X linked and autosomal (Rothwell 1993). Being a recessive trait, the presence of an other X chromosome that is not carrying the recessive i.e. the dominant allele for colour vision, will mask the effect of the recessive. The presence of the second X chromosome is the normal homozygous genotype of females therefore explaining the lower incidence, as two recessives must be present for the phenotype to be exhibited. Therefore a female can be a carrier with out exhibiting the disorder. She would also have a 50/50 chance of passing the recessive chromosome onto any offspring she has. If the recessive is passed onto a hemizygous male offspring he will exhibit the disorder. If she passes it onto a female offspring she will not exhibit the disorder unless her father also carried the recessive (Rothwell 1993). The disorder is still expressed by the heterozygous female who display mosaic retinas with patches of defective colour perception surrounded by normal colour perception a process known as Lyonisation (Klug and Cummings 1996). In the Australian population up to one in seven females are carriers (RAOC 1999). In some isolated populations the incidence is much higher. A genetic bottleneck in the population history may of caused a higher incidence of the disorder. In Hong Kong. The former British Colony, the incidence of colour blindness in the male population was so high that many jobs that many jobs that were restricted to those without the disorder had to accept those with it i.e. the Royal Hong Kong Police Force allowed colour blind constables.
As is written earlier, colour blindness varies between individuals in both the degree of insensitivity and also the wavelengths they are unable to see. There are several different defects and they are named after the Greek word for the primary colours: Protos for red, Deutros for green and Tritos for blue. Some one with a complete red defect is said to have Protoanopia and if he has partial defect they are said to have Protoanomaly. The commonest defect involves the green receptors, they are unable to distinguish red and green, but are sensitive to red light. The next most is failure in the red receptors, these people also confuse red and green but are insensitive to red light. The other defect is due to failure in the blue receptors, a total failure of all receptors and a failure of the rods (RAOC 1999). These are rare and inherited as an autosomal dominant condition that maps to chromosome 7 and is often associated with other disorders (Cummings 1997). The red-green colour deficency misperceptions can be described as follows in: Protanopia: Blue-green appears grey, red-purple appears grey. Protanomalia: Blue-green appears indistinct greyish, red-purple appears indistinct greyish. Deuteranopia: Green appears grey, purple-red appears grey. Deuteranomalia: Green appears indistinct greyish, purple-red appears indistinct greyish. (Oakley 1999).
These are extremes of the different variables, more commonly found are individuals with greater or lesser abilities to distinguish between the various colours and also that the lighting affects the perception of the colours. The mutation is not fixed at any level and severity may vary even within a family. The loci of the protan and deutan genes are known to reside in a band at the tip of the long arm of the X (Xq28) (Rothwell 1993). The genes encode for different forms of Opsins, which are proteins found in the cone cells of the retina. Normal Opsins bind to the visual pigments in the red, green, of blue cone cells. The visual pigment/opsin complex is sensitive to a given wavelength and cause the stimulation of the optic nerves and the perception of then colour/radiation. It the red-opsin gene is absent of defective the function of the red cones are impaired (Cummings 1997). The variations in severity of perception in deuteranomaly (red-green), the most common occurring deficiency, has been examined by a Dr. Jan Neitz, an associate professor at the Medical College of Wisconsin in Milwaukee, U.S.A. A detailed analysis of the DNA of 16 affected men and found that those with the least amount of difference between their red and green photopigment had the most severe colour blindness. Those with a higher degree of difference between the proteins had a less severe form of deuteranomaly. (Reuters 1996).
The prognosis of the disorder is that it is non-fatal, female carriers are not affected and male sufferers are not generally prevented from reaching breeding age and may be neutrally selective genetically appearing to confer no advantage. Therefore evolutionary pressure against the gene is very slight. In a developed world society and culture, it may of in the past and in other cultures of conferred some advantages or disadvantages humans and our primate ancestors. There is a varying level of incidence in other species of our genus, for example squirrel monkeys (Jones et al. 1996) and may convey some advantage. Those individuals who have the disorder have a tendency to have better night vision and an ability to be able to distinguish hues which remain unseen to those who do not have the disorder. The later has been used in wartime by daylight aerial reconnaissance and bombing, as those individuals who were judged to be colour blind were more able to distinguish camouflaged sites than those without the disorder (Birch 1999). In Palaeolithic times, when humanity lived predominately by hunting and gathering colour blindness in females would of been a disadvantage, because the inability to select ripe fruit, i.e. those with an unripe green would suffer (Jones 1993). In males the disorder confers enhanced night vision and therefore improved hunting skills at low levels of light. The incidence of colour deficiencies ranges from 1% to 14% among different indigenous human populations and may be moderately correlated with the amount of twilight. The duration of twilight is short at the equator, but progressively longer at higher latitudes. In the past colour deficient may of had a significant advantage. There is much anecdotal evidence for enhanced visual powers for colour deficients, many other mammals active at twilight are also colour blind. However, the critical experiments on sensitivity to movement and contrast at low light levels have not yet been done (Reimchem 1999).
The diagnosis of the disorder is still based on those principles outlined by Rayleigh. A number of test have been devised, the most basic is to have a number of coloured discs on a screen and ask the patient to identify the colours. The commonest and more efficient test is the use of special test plates called “ pseudo isochromatic” plates or confusion plates. The plates are made up of a series of spots of varying colours and hues so that a central number of letter stands out from the background. Those with defective vision are unable to distinguish these figures or will see a different figure due to the different appreciation of the hues. By changing the colour and hues and the background all basic types of defective colour vision can be identified and classified. Other more specific test can pin point the more subtle defects in colour vision.
The treatment of the disorder by use of somatic gene therapy seems unlikely as the cells that are directly affected are formed in utero and are not in constant replication. The use of gene therapy in order to treat the disorder would be limited because of the inability to target the cells with enough accuracy to ensure that the treatment is effective. The use of vectors does not ensure the correct placement of the gene in to the affected tissue and there are currently fears that the gene therapy would affect the germline tissues causing a permanent hereditary change to the DNA of the individual involved (Boyce March 1999). The nature of the of the disorder would require the gene therapy to be carried out in the early foetal stages of the germline level. The other experimental method, that of ingestion of the DNA in the form of a vaccine would also by of limited use as the nature of the stimuli is in the radiation spectrum and not what is considered physical (Boyce April 1999). The disorder is not fatal and although the sufferers are discriminated against in terms of employment and environment most are able to adapt and carry on reasonably normal lives.
The information gathered about the disorder has very little practical use for society at present except as a vehicle for genetic study. There are still practical applications where the disorder is an advantage, although currently the disadvantages outweigh the benefits. The high numbers of people suffering from the disorder world wide are a minority suffering discrimination as most design and facilities are made with those people who do not suffer from the disorder. Current awareness of the untapped market that colour blind people are, has raised the awareness of designers of things such as website and safety features. They have become aware that colour blind people will not use what they can not see or understand (Oakley 1999).
Colour blindness is not a generally a fatal disorder so very little importance is attached to it, society tends to ignore sufferers or actively discriminate against them.
The genetic information gathered is used largely as a landmark from which other features are identified and is more of passing interest as a tool for the investigation of the rest of the human genome.
References
Bell, J. and Haldene, J. B. S. (1936),The Linkage between the Genes for Colour-blindness and Haemophilia in Man. Proceedings of the Royal Society of London Series B. volume 123 (1937), pages 119-150.
Birch, J. (1999)
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Not seeing Red
http://www.newscientist.com/lastword/answers/1wa151body.html
(12 April 1999)
Boyce, N.
This Week: Fertile is Fine
New Scientist, 20 March 1999 No2178 p22
Boyce, N
This Week: Shock Therapy
New Scientist, 3 April 1999 No 2180 p10
Cummings, M. R. (1997), Human Heredity. Principles and Issues (4th Edition)
West/Wadsworth, International Thomson Publishing Company. New York, London.
Gould, J. L. and Keeton, W. T. (1996)., Biological Science. 6th Edition.
W.W. Norton & Company, New York, London
Henthorne., M. (1999) Homepages:
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(12 April 1999)
Jones, S., (1993). The Language of Genes. Biology, History and Evolutionary Future. Harper Collins Publishers
Jones, S., Martin, R. and Pilbeam, D. (Editors) (1996). The Cambridge Encyclopaedia of Human Evolution. Cambridge University Press, Cambridge
Klug,W. S. and Cummings, M. R. (1996), Essentials of Genetics (2nd Edition)
Prentice-Hall International Inc. New York, London
Oakley, A., (1999)
[WWW]
http://www.cimmerii.demon.co.uk/colourblind
(12 April 1999)
Reimchem, T. E.
New Scientist [WWW]
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http://www.newscientist.com/lastword/answers/1wa151body.html
(12 April 1999)
Reuters (1996)
Reuters 31 October 1996, Health Direct [WWW]
Cause of Color Blindness Discovered
http://www.healthdirect.com/usenew/missedit/mn_color.htm
NB. Referring to an article in Science.
Rothwell, N. V. (1993).Understanding Genetics. A Molecular Approach
Wiley-Liss, Inc. New York.
R. A. O. C., Royal Australian College of Ophthalmologists
Open Publications [WWW]
http://www.raco.org.au/open/publications/colour.htm
(12 April 1999)
