Editing Color blindness (section)

==Causes==
{{See also|Trichromatic color vision|Congenital red–green color blindness#Mechanism}}
Color blindness is any deviation of color vision from normal [[trichromatic]] color vision (often as defined by the [[CIE 1931 color space#CIE standard observer|standard observer]]) that produces a reduced [[gamut]]. Mechanisms for color blindness are related to the functionality of [[cone cell]]s, and often to the expression of [[photopsin]]s, the [[photopigments]] that 'catch' [[photons]] and thereby convert light into chemical signals.

Color vision deficiencies can be classified as inherited or acquired.
* ''Inherited'': inherited or congenital/genetic color vision deficiencies are most commonly caused by mutations of the  genes encoding opsin proteins. However, several other genes can also lead to less common and/or more severe forms of color blindness.
* ''Acquired'': color blindness that is not present at birth, may be caused by chronic illness, accidents, medication, chemical exposure or simply normal aging processes.<ref>
{{cite web
 |title=Acquired colour vision defects
 |website=colourblindawareness.org
 |url=http://www.colourblindawareness.org/colour-blindness/acquired-colour-vision-defects/
 |archive-url=https://web.archive.org/web/20141216094549/http://www.colourblindawareness.org/colour-blindness/acquired-colour-vision-defects/
 |archive-date=2014-12-16
}}
</ref>

===Genetics===
{{unreferenced section|date=May 2023}}

Color blindness is typically an inherited genetic disorder. The most common forms of color blindness are associated with the [[Photopsin]] genes, but the mapping of the human genome has shown there are many causative mutations that do not directly affect the opsins. Mutations capable of causing color blindness originate from at least 19&nbsp;different chromosomes and 56&nbsp;different genes (as shown online at the [[Online Mendelian Inheritance in Man]] [OMIM]).

====Genetics of red–green color blindness====
{{main|Congenital red–green color blindness#Genetics}}
[[File:Punnett square colour blindness.svg|thumb|alt=A chart showing likelihoods of genetic combinations and outcomes for red–green color blindness|Punnett squares for each combination of parents' color vision status giving probabilities of their offsprings' status; A superscript 'c' denotes a chromosome with an affected gene.]]
By far the most common form of color blindness is [[congenital red–green color blindness]] (Daltonism), which includes protanopia/protanomaly and deuteranopia/deuteranomaly. These conditions are mediated by the [[OPN1LW]] and [[OPN1MW]] genes, respectively, both on the [[X chromosome]]. An 'affected' gene is either missing (as in Protanopia and Deuteranopia - [[Dichromacy]]) or is a [[chimeric gene]] (as in Protanomaly and Deuteranomaly).

Since the [[OPN1LW]] and [[OPN1MW]] genes are on the X&nbsp;chromosome, they are [[Sex linkage|sex-linked]], and therefore affect males and females disproportionately. Because the color blind 'affected' [[alleles]] are recessive, color blindness specifically follows [[X-linked recessive inheritance]]. Males have only one X&nbsp;chromosome (XY), and females have two (XX); Because the male only has one of each gene, if it is affected, the male will be color blind. Because a female has two alleles of each gene (one on each chromosome), if only one gene is affected, the dominant normal alleles will "override" the affected, recessive allele and the female will have normal color vision. However, if the female has two mutated alleles, she will still be color blind. This is why there is a disproportionate prevalence of color blindness, with ~8% of males exhibiting color blindness and ~0.5% of females.

====Genetics of blue–yellow color blindness====
Congenital blue–yellow color blindness is a much rarer form of color blindness including tritanopia/tritanomaly. These conditions are mediated by the [[OPN1SW]] gene on [[Chromosome 7]] which encodes the S-opsin protein and follows autosomal dominant inheritance.<ref name="Sharpe1999"/> The cause of blue–yellow color blindness is not analogous to the cause of red–green color blindness, i.e. the peak sensitivity of the S-opsin does not shift to longer wavelengths. Rather, there are 6 known point mutations of OPN1SW that degrade the performance of the S-cones.<ref name="RCM2020">{{cite book |last1=Rodriguez-Carmona |first1=Marisa |last2=Patterson |first2=Emily J. |chapter=Photoreceptors, Color Vision |title=Encyclopedia of Color Science and Technology |date=2020 |pages=1–7 |doi=10.1007/978-3-642-27851-8_277-3 |isbn=978-3-642-27851-8 |s2cid=226504635 |chapter-url=https://openaccess.city.ac.uk/id/eprint/23584/1/Photoreceptors%20Color%20Vision_submitted%20to%20CRO.pdf |access-date=18 December 2023 |archive-date=2 December 2023 |archive-url=https://web.archive.org/web/20231202184422/https://openaccess.city.ac.uk/id/eprint/23584/1/Photoreceptors%20Color%20Vision_submitted%20to%20CRO.pdf |url-status=live }}</ref> The OPN1SW gene is almost invariant in the human population. Congenital tritan defects are often progressive, with nearly normal trichromatic vision in childhood (e.g. mild tritanomaly) progressing to dichromacy (tritanopia) as the S-cones slowly die.<ref name="RCM2020"/> Tritanomaly and tritanopia are therefore different penetrance of the same disease, and some sources have argued that tritanomaly therefore be referred to as incomplete tritanopia.<ref name="Sharpe1999">{{cite book |last1=Sharpe |first1=LT |last2=Stockman |first2=A |last3=Jägle |first3=H |last4=Nathans |first4=J |title=Color vision: From genes to perception |date=1999 |page=351 |url=http://www.cvrl.org/people/stockman/pubs/1999%20Genetics%20chapter%20SSJN.pdf |chapter=Opsin genes, cone photopigments, color vision, and color blindness. |quote=True cases of tritanomaly, as distinct from partial or incomplete tritanopia, have never been satisfactorily documented. Although the separate existence of tritanopia and tritanomaly, with different modes of inheritance, has been postulated, it now seems more likely that tritanomaly does not exist, but rather has been mistaken for incomplete tritanopia. |access-date=16 December 2023 |archive-date=3 October 2024 |archive-url=https://web.archive.org/web/20241003080812/http://www.cvrl.org/people/stockman/pubs/1999%20Genetics%20chapter%20SSJN.pdf |url-status=live }}</ref>

====Other genetic causes====
Several inherited diseases are known to cause color blindness, including [[achromatopsia]], [[cone dystrophy]], [[Leber's congenital amaurosis]] and [[retinitis pigmentosa]]. These can be [[congenital]] or commence in childhood or adulthood. They can be static/stationary or [[Progressive disease|progressive]]. Progressive diseases often involve deterioration of the retina and other parts of the eye, so often progress from color blindness to more severe [[visual impairment]]s, up to and including total blindness.

===Non-genetic causes===
Physical trauma can cause color blindness, either neurologically – brain trauma which produces swelling of the brain in the [[occiput|occipital lobe]] – or retinally, either acute (e.g. from laser exposure) or chronic (e.g. from [[ultraviolet light]] exposure).

Color blindness may also present itself as a symptom of degenerative diseases of the eye, such as [[cataract]] and age-related [[macular degeneration]], and as part of the retinal damage caused by [[diabetes]]. [[Vitamin A]] deficiency may also cause color blindness.<ref>
{{cite book
 |editor1-last=Leikin  |editor1-first=Jerrold B.
 |editor2-last=Lipsky  |editor2-first=Martin S.
 |year=2003
 |title=Complete Medical Encyclopedia  |edition=First
 |page=[https://archive.org/details/americanmedicala00amer/page/388 388]
 |publisher=Random House, for the American Medical Association
 |location=New York, NY
 |series=Random House Reference
 |isbn=978-0-8129-9100-0
 |via=archive.org
 |url=https://archive.org/details/americanmedicala00amer
 |access-date=1 December 2011 |url-access=registration
}}
</ref>

Color blindness may be a [[side effect]] of prescription drug use. For example, red–green color blindness can be caused by [[ethambutol]], a drug used in the treatment of [[tuberculosis]].<ref>
{{cite web |last=Unknown |first=Unknown |title=NEI - Types of Color Vision Deficiency |url=https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/color-blindness/types-color-vision-deficiency |url-status=live |archive-url=https://web.archive.org/web/20140708223716/http://www.rxlist.com/myambutol-drug.htm |archive-date=2014-07-08 |access-date=2024-07-21 |website=www.nei.nih.gov |series=Color Blindness, Color Vision Deficiency |quote=Description, Types of Color Vision Deficiency,}}
</ref> Blue–yellow color blindness can be caused by [[sildenafil]], an active component of [[Viagra]].<ref>{{cite web
 |title=Viagra (Sildenafil Citrate) Drug
 |series=drug information
 |website=RxList.com
 |quote=Description, user reviews, drug side effects, interactions–prescribing information
 |url=http://www.rxlist.com/viagra-drug.htm
 |access-date=2022-06-03
 |archive-date=8 June 2022
 |archive-url=https://web.archive.org/web/20220608010719/https://www.rxlist.com/viagra-drug.htm
 |url-status=live
 }}</ref> [[Hydroxychloroquine]] can also lead to [[hydroxychloroquine]] retinopathy, which includes various color defects.<ref>{{cite book
 |last1=Fraunfelder
 |first1=Frederick T.
 |last2=Fraunfelder
 |first2=Frederick W.
 |last3=Chambers
 |first3=Wiley A.
 |year=2014
 |title=Drug-Induced Ocular Side Effects: Clinical ocular toxicology e‑book
 |page=79
 |publisher=Elsevier Health Sciences
 |isbn=978-0-323-31985-0
 |url=https://books.google.com/books?id=6bqXBAAAQBAJ&pg=PA79
 |language=en
 |access-date=19 March 2023
 |archive-date=3 October 2024
 |archive-url=https://web.archive.org/web/20241003080813/https://books.google.com/books?id=6bqXBAAAQBAJ&pg=PA79#v=onepage&q&f=false
 |url-status=live
 }}</ref> Exposure to chemicals such as [[styrene]]<ref>
{{cite journal
 |vauthors = Choi AR, Braun JM, Papandonatos GD, Greenberg PB
 |date = November 2017
 |title = Occupational styrene exposure and acquired dyschromatopsia: A systematic review and meta-analysis
 |journal = American Journal of Industrial Medicine
 |volume = 60 |issue = 11 |pages = 930–946
 |pmid = 28836685 |pmc = 5652067
 |doi = 10.1002/ajim.22766
}}
</ref> or organic solvents<ref>
{{cite journal
 |vauthors = Betancur-Sánchez AM, Vásquez-Trespalacios EM, Sardi-Correa C
 |date = January 2017
 |title = Impaired colour vision in workers exposed to organic solvents: A systematic review
 |journal = Archivos de la Sociedad Espanola de Oftalmologia
 |volume = 92 |issue = 1 |pages = 12–18
 |pmid = 27422480 |doi = 10.1016/j.oftal.2016.05.008
}}
</ref><ref name="Dick">
{{cite journal
 | last = Dick  |first = F.D.
 | date = March 2006
 | title = Solvent neurotoxicity
 | journal = Occupational and Environmental Medicine
 | volume = 63 | issue = 3 | pages = 221–6, 179
 | pmid = 16497867 | pmc = 2078137 | doi = 10.1136/oem.2005.022400
}}
</ref> can also lead to color vision defects.

Simple colored filters can also create mild color vision deficiencies. John Dalton's original hypothesis for his deuteranopia was actually that the [[vitreous humor]] of his eye was discolored:

{{Blockquote
|text=I was led to conjecture that one of the humours of my eye must be a transparent, but coloured, medium, so constituted as to absorb red and green rays principally... I suppose it must be the vitreous humor.
|author=John Dalton
|source=''Extraordinary facts relating to the vision of colours: with observations'' (1798)
}}

An autopsy of his eye after his death in 1844 showed this to be definitively untrue,<ref>{{cite journal |last1=Hunt |first1=D. |last2=Dulai |first2=K. |last3=Bowmaker |first3=J. |last4=Mollon |first4=J. |title=The Chemistry of John Dalton's Color Blindness |journal=Science |date=February 17, 1995 |volume=267 |issue=5200 |pages=984–988 |doi=10.1126/science.7863342|pmid=7863342 |bibcode=1995Sci...267..984H |s2cid=6764146 }}</ref> though other filters are possible. Actual physiological examples usually affect the blue–yellow opponent channel and are named [[Cyanopsia]] and [[Xanthopsia]], and are most typically an effect of yellowing or removal of the [[Lens (anatomy)|lens]].

The opponent channels can also be affected by the prevalence of certain cones in the [[retinal mosaic]]. The cones are not equally prevalent and not evenly distributed in the retina. When the number of one of these cone types is significantly reduced, this can also lead to or contribute to a color vision deficiency. This is one of the causes of '''tritanomaly'''.

Some people are also unable to distinct between blue and green, which appears to be a combination of [[Cultural neuroscience#Culture differences in visual stimuli|culture]] and exposure to UV-light.<ref>{{Cite journal |last1=Josserand |first1=Mathilde |last2=Meeussen |first2=Emma |last3=Majid |first3=Asifa |last4=Dediu |first4=Dan |date=2021 |title=Environment and culture shape both the colour lexicon and the genetics of colour perception |journal=Scientific Reports |language=en |volume=11 |issue=1 |pages=19095 |doi=10.1038/s41598-021-98550-3 |pmid=34580373 |pmc=8476573 |bibcode=2021NatSR..1119095J |issn=2045-2322}}</ref>