Congenital red–green color blindness is an inherited condition that is the root cause of the majority of cases of
color blindness
Color blindness or color vision deficiency (CVD) is the decreased ability to color vision, see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may ...
. It has no significant symptoms aside from its minor to moderate effect on
color vision
Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different wavelengths (i.e., different spectral power distributions) independently of light intensity. Color perception is a part of ...
.
[ It is caused by variation in the functionality of the red and/or green ]opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most pro ...
proteins, which are the photosensitive pigment in the cone cell
Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cone ...
s of the retina, which mediate color vision.[ Males are more likely to inherit red–green color blindness than females, because the ]genes
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
for the relevant opsins are on the X chromosome
The X chromosome is one of the two sex-determining chromosomes (allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO sex-d ...
.[ Screening for congenital red–green color blindness is typically performed with the Ishihara or similar ]color vision test
A color vision test is used for measuring color vision against a standard. These tests are most often used to diagnose color vision deficiencies (''color blindness''), though several of the standards are designed to categorize normal color vision ...
.[ There is no cure for color blindness.]
This form of colorblindness is sometimes referred to historically as daltonism after John Dalton
John Dalton (; 5 or 6 September 1766 – 27 July 1844) was an English chemist, physicist and meteorologist. He is best known for introducing the atomic theory into chemistry, and for his research into colour blindness, which he had. Colour b ...
, who had congenital red–green color blindness and was the first to scientifically study it. In other languages, ''daltonism'' is still used to describe red–green color blindness, but may also refer colloquially to color blindness
Color blindness or color vision deficiency (CVD) is the decreased ability to color vision, see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may ...
in general.
Symptoms
The only significant symptom of congenital red–green color blindness is deficient color vision (color blindness or discromatopsia). Additionally, approximately __% of colorblindness is caused by congenital red–green colorblindness, so the condition and symptom are often difficult to untangle. A red–green color blind subject will have decreased (or no) color discrimination along the red–green axis. This commonly includes the following colors of confusion:
* Cyan and Grey
* Rose-Pink and Grey
* Blue and Purple
* Yellow and Neon Green
* Red, Green, Orange, Brown
* Black and Red (protans)
Classification
Congenital red–green color blindness is classified into 1 of 4 groups:
* Protanopia
* Protanomaly
* Deuteranopia
* Deuteranomaly
Each of these groups comprises a prefix and a suffix. The prefix indicates the cone (photopsin
Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins discovered and are still th ...
) that is affected, with lexemes from Greek, "first" (''prot-'') or "second" (''deuter-'') referring to the L- and M-opsins respectively. The suffix indicates the dimensionality of color vision:
* Dichromacy
Dichromacy (from Greek ''di'', meaning "two" and ''chromo'', meaning "color") is the state of having two types of functioning photoreceptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats require ...
gives the suffix ''-anopia'' (from the Greek for "no sight")
* Anomalous trichromacy
Color blindness or color vision deficiency (CVD) is the decreased ability to color vision, see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may ...
gives the suffix ''anomaly'' (from the Greek for "irregular").
Dimensionality
The dimensionality of normal color vision is trichromatic
Trichromacy or trichromatism is the possessing of three independent channels for conveying color information, derived from the three different types of cone cells in the eye. Organisms with trichromacy are called trichromats.
The normal expl ...
. This references that a visual system with three distinct cone classes and therefore a three dimensional gamut
In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain ''complete subset'' of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circ ...
. Dichromatic color vision only has two distinct cone classes and therefore a two dimensional gamut. With red–green dichromacy, it is the dimension representing the red–green opponent channel that is lost. Anomalous trichromacy is also trichromatic, but the spectral sensitivity
Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal.
In visual neuroscience, spectral sensitivity is used to describe the different characteristics o ...
of at least one of the cone cells is altered, leading to a gamut that is a different size or shape. In the case of congenital red–green color blindness, the dynamic range
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base-1 ...
of the red–green dimension is decreased when compared to normal color vision.
The dimensionality of the defect is related to the strength/severity, but it is usually much easier clinically to define the severity empirically as mild, moderate and strong (or severe). Anomalous trichromacy can vary in severity from indistinguishable from normal color vision (mild) to indistinguishable from dichromacy (strong). Therefore, the differential diagnosis between anomalous trichromacy and dichromacy is difficult. An example clinical diagnosis would be ''strong deutan'', which could correspond to ''either'' deuteranomaly or deuteranopia.
Protan vs. deutan
The two types of congenital red–green color blindness as based on the affected cone are:
* Protan: (2% of males): lacking, or possessing anomalous L-opsins for long-wavelength-sensitive cone cells.
* Deutan: (6% of males): lacking, or possessing anomalous M-opsins for medium-wavelength-sensitive cone cells.
Despite often being called red-blind and green-blind respectively, protan and deutan varieties have very similar phenotypes (color vision), especially when compared to tritan color blindness. The condition is not called red–green color blindness because red and green are indicative colors of confusion, nor because the "red" and "green" cones are affected, but because the red–green opponent process
The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from photoreceptor cells in an antagonistic manner. The opponent-process theory suggests that there are thr ...
channel is affected. In dichromacy, that channel is equally deactivated regardless of which cone (LWS or MWS) is missing. In anomalous trichromacy, that channel is equally affected regardless of which cone effectively moves towards the other.
The starkest difference is the scoterythrous effect, where reds appear dimmer to protans. This is why protans often confuse red with black, while deutans do not. The protan luminous efficiency function
A luminous efficiency function or luminosity function represents the average spectral sensitivity of human visual perception of light. It is based on subjective judgements of which of a pair of different-colored lights is brighter, to describe ...
is narrower at long wavelengths, which causes the reds to be darker. This is due to the red cones (which normally cover the red side of the spectrum) either shifting to shorter wavelengths or being missing.
The two are difficult to differentiate with color vision test
A color vision test is used for measuring color vision against a standard. These tests are most often used to diagnose color vision deficiencies (''color blindness''), though several of the standards are designed to categorize normal color vision ...
s, but is most reliably performed with an Anomaloscope
An anomaloscope is an instrument and color vision test, often used to quantify and characterize color blindness. They are expensive and require specialized knowledge to operate, but are viewed as the gold standard for color vision standards. As a ...
. This device measures the proportion of red and green light that must be mixed to perceptually match a yellow reference. Protans add more red than color normals, and deutans add more green.
Mechanism
Genes
The mechanism of congenital red–green color blindness relates to the functionality of cone cell
Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cone ...
s, specifically to the expression of photopsin
Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins discovered and are still th ...
s, the photopigments
Photopigments are unstable pigments that undergo a chemical change when they absorb light. The term is generally applied to the non-protein chromophore moiety of photosensitive chromoproteins, such as the pigments involved in photosynthesis and pho ...
that 'catch' photons
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alway ...
and thereby convert light into chemical signals. A typical human has three distinct photopsins: S-, M- and L-opsins expressed by distinct genes, respectively OPN1SW
Blue-sensitive opsin is a protein that in humans is encoded by the ''OPN1SW'' gene.
See also
* Opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound t ...
, OPN1MW
Green-sensitive opsin is a protein that in humans is encoded by the ''OPN1MW'' gene.
OPN1MW2
OPN1MW2 is a duplication of the OPN1MW gene, which encodes the medium wavelength sensitive (MWS) photopsin. The gene duplication is present in about 50% ...
or OPN1LW
OPN1LW is a gene on the X chromosome that encodes for long wave sensitive (LWS) opsin, or red cone photopigment. It is responsible for perception of visible light in the yellow-green range on the visible spectrum (around 500-570nm). The gene conta ...
. OPN1MW and OPN1LW are located in a gene cluster
A gene family is a set of homologous genes within one organism. A gene cluster is a group of two or more genes found within an organism's DNA that encode similar polypeptides, or proteins, which collectively share a generalized function and are o ...
(along with a locus control region gene) at position
Position often refers to:
* Position (geometry), the spatial location (rather than orientation) of an entity
* Position, a job or occupation
Position may also refer to:
Games and recreation
* Position (poker), location relative to the dealer
* ...
Xq28, at the end of the q arm of the X chromosome in a tandem array
Tandemly arrayed genes (TAGs) are a gene cluster created by tandem duplications, a process in which one gene is duplicated and the copy is found adjacent to the original. They serve to encode large numbers of genes at a time.
TAGs represent a larg ...
. OPN1SW is unrelated to the condition and located on a different chromosome. The genes in the cluster are summarized in the following table:
Differentiating from a duplication event 30-40 MYA, the two opsins are highly homologous (very similar), having only 19 dimorphic sites (amino acids that differ), and are therefore 96% similar.[ In comparison, either of these opsin genes are only 40% homologous to ]OPN1SW
Blue-sensitive opsin is a protein that in humans is encoded by the ''OPN1SW'' gene.
See also
* Opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound t ...
(encoding the SWS photopsin
Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins discovered and are still th ...
and located on chromosome 7
Chromosome 7 is one of the 23 pairs of chromosomes in humans, who normally have two copies of this chromosome. Chromosome 7 spans about 159 million base pairs (the building material of DNA) and represents between 5 and 5.5 percent of the total DN ...
) or ''"RHO"'' (encoding rhodopsin
Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction ...
, and located on chromosome 3
Chromosome 3 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 3 spans almost 200 million base pairs (the building material of DNA) and represents about 6.5 percent of the total DNA in ...
).
While the two genes share 19 dimorphic sites (amino acids that differ), only 7 of them lead to a functional difference between the genes, i.e. tune the opsin's spectral sensitivity
Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal.
In visual neuroscience, spectral sensitivity is used to describe the different characteristics o ...
.[ These 7 functionally dimorphic sites will tune the opsin to a higher (]redshift
In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in f ...
) or lower (blueshift
In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in fr ...
) wavelength. The typical (most common) allele for the OPN1MW gene is blueshifted at every one of these dimorphic sites. Likewise, the typical allele for the OPN1LW gene is redshifted at every one of these dimorphic sites. In other words, the most common alleles of each gene that contribute to normal color vision are as far as part as they can be in the spectrum (about 30 nm) without novel point mutation
A point mutation is a genetic mutation where a single nucleotide base is changed, inserted or deleted from a DNA or RNA sequence of an organism's genome. Point mutations have a variety of effects on the downstream protein product—consequences ...
s.
Homologous recombination
During meiosis, homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...
between chromosomes of the same type may occur where they exchange a portion of their genes. The exchanged portions are generally equivalent (have the same genes) and this process is called ''equal'' homologous recombination. ''Unequal'' homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...
occurs when the exchanged portions of the chromosomes are not equal, i.e. they don't break in the same spot. This recombination happens often at this locus because the OPN1LW and OPN1MW genes are adjacent and 96% similar.
When unequal recombination happens with breaks between the genes (depicted by blue lines), a gene can be essentially deleted from one of the chromosomes. This gene deletion leads to protanopia or deuteranopia (congenital red–green dichromacy).
When unequal recombination happens with breaks in the ''middle'' of a gene (e.g. between exon
An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequen ...
s), chimeric genes can be created that contain portions of each of the OPN1LW/OPN1MW genes.
Chimeric gene
A chimeric gene contains exons contributed from the typical alleles of each of the OPN1MW and OPN1LW genes. Due to the similarity between the genes, these chimeras are always functional, but experience a spectral tuning, i.e. a change to the spectral sensitivity. A chimera's spectral sensitivity will lie somewhere between the typical allele peaks (530~560 nm). These chimeric alleles are indicated with an asterisk, either M* or L*. Whether a chimeric gene is described as M* or L* is not based on whether they are closer in the spectrum to the typical M or L allele, but rather is the opposite of the other expressed gene. This means the same chimeric gene could be labeled M* or L* depending on what other genes are in the gene cluster. An individual with protanomaly will have M and L* opsins and an individual with deuteranomaly will have L and M* opsins.
The following table includes the 7 dimorphic sites that contribute to spectral tuning, including their exon and the spectral shift between the typical OPN1MW and OPN1LW exons (as a whole):
Gene duplications
Gene duplications are one result of the unequal homologous recombination. Either the OPN1LW
OPN1LW is a gene on the X chromosome that encodes for long wave sensitive (LWS) opsin, or red cone photopigment. It is responsible for perception of visible light in the yellow-green range on the visible spectrum (around 500-570nm). The gene conta ...
or OPN1MW
Green-sensitive opsin is a protein that in humans is encoded by the ''OPN1MW'' gene.
OPN1MW2
OPN1MW2 is a duplication of the OPN1MW gene, which encodes the medium wavelength sensitive (MWS) photopsin. The gene duplication is present in about 50% ...
can be duplicated, though it is much more commonly the latter. Only 5% of X chromosomes contain multiple OPN1LW genes, but 55% contain multiple OPN1MW genes, sometimes as many as 4. The duplicate genes are sometimes referred to with numerical suffixes, where the OPN1MW gene in the second position is called OPN1MW2
OPN1MW2 is a duplication of the OPN1MW gene, which encodes the medium wavelength sensitive (MWS) photopsin. The gene duplication is present in about 50% of X-chromosomes, so is present in 50% of males and at least once 75% of females. It caused by ...
. The duplicated genes are always in sequence and can consist of different alleles of the gene, but only the first gene of a duplicate series is ever expressed.
Blue-cone monochromacy
While blue-cone monochromacy exhibits much stronger symptoms than the congenital red–green color blindness (including total color blindness), it follows a very similar mechanism. In most cases, unequal homologous combination must first occur to generate a genotype with a single L/M-opsin gene. Then that gene must experience a nonsense mutation
In genetics, a nonsense mutation is a point mutation in a sequence of DNA that results in a premature stop codon, or a ''nonsense codon'' in the transcribed mRNA, and in leading to a truncated, incomplete, and usually nonfunctional protein produc ...
to entirely deactivate it.
Genetics
''Congenital'' means that the condition is present from birth, but is usually used to represent the genetic, inherited basis of the condition. This is in contrast to ''acquired'' color blindness that is not present at birth and may be caused by aging, accidents, medication, etc.
Heredity
Since the affected opsin genes (OPN1LW
OPN1LW is a gene on the X chromosome that encodes for long wave sensitive (LWS) opsin, or red cone photopigment. It is responsible for perception of visible light in the yellow-green range on the visible spectrum (around 500-570nm). The gene conta ...
and OPN1MW
Green-sensitive opsin is a protein that in humans is encoded by the ''OPN1MW'' gene.
OPN1MW2
OPN1MW2 is a duplication of the OPN1MW gene, which encodes the medium wavelength sensitive (MWS) photopsin. The gene duplication is present in about 50% ...
) are on the X chromosome, they are sex-linked
Sex linked describes the sex-specific patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome ( autosome). In humans, these are termed X-linked recessive, ...
, and therefore affect males and females disproportionately. Because the colorblind alleles
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
are recessive, colorblindness follows X-linked recessive inheritance
X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males (who are necessarily homozygous for the gene mutation because they have one X and one Y ...
. Males have only one X chromosome ( karyotype XY), and females have two (karyotype XX); Because the male only has one allele of each gene, if it is missing or chimeric, the male will be color blind. Because a female has two alleles of each gene (one on each chromosome), if only one allele is mutated, the dominant normal alleles will "override" the mutated, recessive allele and the female will have normal color vision. However, if the female has two mutated alleles, she will still be colorblind. This is why there is a disproportionate prevalence of colorblindness, with ~8% of males exhibiting colorblindness and ~0.5% of females
Some conclusions from the table include:
* A male cannot inherit colorblindness from his father.
* A colorblind female ''must'' have a colorblind father.
* A female must inherit colorblindness alleles from both parents to be colorblind.
* Colorblind females can ''only'' produce colorblind males.
* Because carrier females often have a colorblind father, colorblind males often will have a colorblind maternal grandfather (or great-grandfather). In this way, colorblindness is often said to 'skip a generation'.
The Punnett square and this section assume each chromosome only has one affected gene. It also assumes females with two affected chromosomes are affected in the same way.
Genotypes
The table to the right shows the possible allele/chromosome combinations and how their interactions will manifest in an individual. The exact phenotype of some of the combinations depend on whether the affected gene represents an anomalous allele or is missing. For example, the XY male may have blue-cone monochromacy if the genes are both missing/non-functional, or near-normal color vision if both genes are anomalous.
* Y : male-only chromosome (no effect on colorblindness)
* X : X chromosome will have two subscripts indicating the alleles present:
** M : normal M opsin allele
** L : normal L opsin allele
** M*: chimeric (or missing) M opsin allele
** L*: chimeric (or missing) L opsin allele
Tetrachromacy in carriers of CVD
Females that are heterozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mo ...
for anomalous trichromacy (i.e. carrier
Carrier may refer to:
Entertainment
* ''Carrier'' (album), a 2013 album by The Dodos
* ''Carrier'' (board game), a South Pacific World War II board game
* ''Carrier'' (TV series), a ten-part documentary miniseries that aired on PBS in April 20 ...
s) may be tetrachromats. These females have two alleles for either the OPN1MW
Green-sensitive opsin is a protein that in humans is encoded by the ''OPN1MW'' gene.
OPN1MW2
OPN1MW2 is a duplication of the OPN1MW gene, which encodes the medium wavelength sensitive (MWS) photopsin. The gene duplication is present in about 50% ...
or OPN1LW
OPN1LW is a gene on the X chromosome that encodes for long wave sensitive (LWS) opsin, or red cone photopigment. It is responsible for perception of visible light in the yellow-green range on the visible spectrum (around 500-570nm). The gene conta ...
gene, and therefore express both the normal and anomalous opsins. Because one X chromosome is inactivated at random in each photoreceptor cell
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiatio ...
during a female's development, those normal and anomalous opsins will be segregated into their own cone cells, and because these cells have different spectral sensitivity
Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal.
In visual neuroscience, spectral sensitivity is used to describe the different characteristics o ...
, they can functionally operate as different cone classes. This theoretical female would therefore have cones with peak sensitivities at 420 nm (S cone), 530 nm (M cone), 560 nm (L cone) and the fourth (anomalous) cone between 530 nm and 560 nm (either M* or L* cone).
If a female is heterozygous for ''both'' protanomaly and deuteranomaly, she could be pentachromatic. The degree to which women who are carriers of either protanomaly or deuteranomaly are demonstrably tetrachromatic and require a mixture of four spectral lights to match an arbitrary light is very variable. Jameson ''et al.'' have shown that with appropriate and sufficiently sensitive equipment it can be demonstrated that any female carrier of red–green color blindness (i.e. heterozygous protanomaly, or heterozygous deuteranomaly) is a tetrachromat to a greater or lesser extent.
Since the incidence of anomalous trichromacy in males is ~6%, which should equal the incidence of anomalous M opsin or L opsin alleles, it follows that the prevalence of unaffected female carriers of colorblindness (and therefore of potential tetrachromats) is 11.3% (i.e. 94% × 6% × 2), based on the Hardy–Weinberg principle
In population genetics, the Hardy–Weinberg principle, also known as the Hardy–Weinberg equilibrium, model, theorem, or law, states that allele and genotype frequencies in a population will remain constant from generation to generation in t ...
. One such woman has been widely reported to be a true or functional tetrachromat, as she can discriminate colors most other people can't.
Diagnosis
Color vision test
The diagnosis of congenital red–green color blindness is usually inferred through psychophysical testing. These color vision tests test detect the color vision phenotype, and not the subject genotype, so are unable to differentiate acquired from congenital red–green color blindness. However, color vision and genotype are highly correlated, especially when acquired color blindness is ruled out.[ The ]Ishihara color test
The Ishihara test is a color vision test for detection of red-green color deficiencies. It was named after its designer, Shinobu Ishihara, a professor at the University of Tokyo, who first published his tests in 1917.S. Ishihara, Tests for colo ...
is the test most often used to detect red–green deficiencies and most often recognized by the public.
Electroretinography
When psychophysical testing is undesired, an electroretinogram
Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors ( rods and cones), inner retinal cells ( bipolar and amacrine cells), and the ganglion cells. Electrodes are placed on the ...
(ERG) can be used instead. An ERG measures the electrical response from the retina as a function of wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of light. Due to the shape of spectral sensitivity
Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal.
In visual neuroscience, spectral sensitivity is used to describe the different characteristics o ...
of cone cell
Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cone ...
s, the peak wavelengths of cone sensitivity can be assumed from an ERG. The peak wavelengths are highly correlated to genotype.
Genetic testing
The genotype
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
can be directly evaluated by sequencing
In genetics and biochemistry, sequencing means to determine the primary structure (sometimes incorrectly called the primary sequence) of an unbranched biopolymer. Sequencing results in a symbolic linear depiction known as a sequence which succ ...
the OPN1MW
Green-sensitive opsin is a protein that in humans is encoded by the ''OPN1MW'' gene.
OPN1MW2
OPN1MW2 is a duplication of the OPN1MW gene, which encodes the medium wavelength sensitive (MWS) photopsin. The gene duplication is present in about 50% ...
and OPN1LW
OPN1LW is a gene on the X chromosome that encodes for long wave sensitive (LWS) opsin, or red cone photopigment. It is responsible for perception of visible light in the yellow-green range on the visible spectrum (around 500-570nm). The gene conta ...
genes. The correlation between genotype
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
and phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
(color vision) are well known, so genetic testing can be a useful supplement to psychophysical color vision test
A color vision test is used for measuring color vision against a standard. These tests are most often used to diagnose color vision deficiencies (''color blindness''), though several of the standards are designed to categorize normal color vision ...
s that may provide incomplete information.
Treatment
Despite much recent improvement in Gene therapy for color blindness Gene therapy for color blindness is an experimental gene therapy of the human retina aiming to grant typical trichromatic color vision to individuals with congenital color blindness by introducing typical alleles for opsin
Animal opsins are G- ...
, there is currently no FDA approved treatment for congenital red–green color blindness, and otherwise no cure for exists. Management of the condition through the use of color blind glasses
Color blind glasses or color correcting lenses are light filters, usually in the form of glasses or contact lenses, that attempt to alleviate color blindness, by bringing deficient color vision closer to normal color vision or to make certain co ...
to alleviate symptoms or smartphone apps to aid with daily tasks is possible.
Epidemiology
Congenital red–green color blindness affects a large number of individuals, especially individuals of European ancestry, where 8% of men and 0.4% of women exhibit congenital red–green color deficiency. The lower prevalence in females is related to the x-linked inheritance of congenital red–green color blindness, as explained above. Interestingly, even Dalton's very first paper already arrived upon this 8% number:
Other ethnicities will generally have a lower prevalence of congenital red–green color blindness. The following table summarizes a number of studies performed in different regions.
See also
* List of people with color blindness
This is a list of people with color blindness, meaning they have a decreased ability to see color or differences in color.
A–Z
References
{{reflist, colwidth=30em
People
A person ( : people) is a being that has certain capacities ...
* Red–green color space
* Tetrachromacy
Tetrachromacy (from Greek language, Greek ''tetra'', meaning "four" and ''chromo'', meaning "color") is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cell in the eye. Org ...
References
Further reading
*
*
*
{{Authority control
Color vision
Genetic disorders by system
Visual disturbances and blindness
X-linked recessive disorders