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Classification
Incidence (%)
Males
Females
Anomalous
Trichromacy6.3
0.37
Protanomaly
(L-cone defect)1.3
0.02
Deuteranomaly
(M-cone defect)5.0
0.35
Tritanomaly
(S-cone defect)0.0001
0.0001
Dichromacy
2.4
0.03
Protanopia
(L-cone absent)1.3
0.02
Deuteranopia
(M-cone absent)1.2
0.01
Tritanopia
(S-cone absent)0.001
0.03
Rod Monochromacy
(no cones)0.00001
0.00001
Inheritance & incidence
Most kinds of congenital colorblindness are caused by defects in the X chromosome. Since females have two X chromosomes and males have an X chromosome and a Y chromosome, colorblindness is much more common in males: females must have defects in both chromosomes before they exhibit colorblindness.
A female with the colorblindness defect in one X chromosome is a carrier of colorblindness. Male children of a female carrier are as likely to be colorblind as male children of a male with colorblindness, and male children of a male with colorblindness and a female carrier are extremely likely to be colorblind.
Approximately 5% to 8% of the men and 0.5% of the women of the world are born colorblind. That's as high as one out of twelve men and one out of two hundred women. People who are protans (red weak) and deutans (green weak) comprise 99% of this group.
Molecular genetics
The M- and L-cone photopigment genes lie in a head to tail tandem array on the q-arm of the X-chromosome.
Colorblindness was mapped to the
X chromosome by its inheritance pattern in 1911.Each gene consists of six coding regions, called exons, which are transcribed to produce the opsin. Because the M- and L-cone photopigment genes are highly homologous and adjacent to one another, intragenic recombination between them is common and can lead to the production of hybrid or fusion genes, some of which code for anomalous pigments. Each hybrid gene can be identified by the site, usually between exons, at which the fusion occurs. For example, L3M4 indicates a hybrid gene in which exons 1 to 3 derive from an L-cone pigment gene and exons 4 to 6 from an M-cone pigment gene. Because exons 1 and 6 in the L- and M-cone pigment genes are identical, a L1M2 hybrid pigment gene encodes a de facto M-cone photopigment.
Polymorphisms is common in the normal population, such as the frequent substitution of alanine by serine at codon 180 in exon 3. For example, among Caucasian males, researchers estimate that 56% have the serine variant [identified as L(S180)] and 44% the alanine variant [identified as L(A180)] for their L-cone gene.
People with Anomalous Trichromacy
Anomalous trichromacy is characterized by a shift in the sensitivity of one or more cone types. For reasons having to do with the genetic coding of color vision, these shifts overwhelmingly happen in the L-cone and M-cone, and the shift is typically from the one toward the other. In other words, protanomaly makes the L-cone more like the M-cone, and deutanomaly makes the M-cone more like the L-cone.
Anomalous trichromacy is by far the most common form of colorblindness, and in many senses it is the least severe. Its effect is to reduce the ability to discriminate between colors, but it does not eliminate color perception altogether.
Deutanomaly and protanomaly together are popularly called "red-green" colorblindness because the sensitivity shift typically reduces the ability to distinguish those two colors.
Under poor viewing conditions, such as when driving in dazzling sunlight or in rainy or foggy weather, it is easily possible for protanomalous individuals to mistake a blinking red traffic light from a blinking yellow or amber one, or to fail to distinguish a green traffic light from the various "white" lights in store fronts, signs, and street lights that line our streets.
Protanomaly (one out of 100 males)
Protanomaly is referred to as "red-weakness," an apt description of this form of color deficiency. Any redness seen in a color by a normal observer is seen more weakly by the protanomalous viewer, both in terms of its "coloring power" (saturation, or depth of color) and its brightness. Red, orange, yellow, yellow-green, and green, appear somewhat shifted in hue ("hue" is just another word for "color") towards green, and all appear paler than they do to the normal observer. The redness component that a normal observer sees in a violet or lavender color is so weakened for the protanomalous observer that he may fail to detect it, and therefore sees only the blue component. Hence, to him the color that normals call "violet" may look only like another shade of blue.
Deuteranomaly (five out of 100 males)
The deuteranomalous person is considered "green weak." Similar to the protanomalous person, he is poor at discriminating small differences in hues in the red, orange, yellow, green region of the spectrum. He makes errors in the naming of hues in this region because they appear somewhat shifted towards red for him. One very important difference between deuteranomalous individuals and protanomalous individuals is deuteranomalous individuals do "not" have the loss of "brightness" problem.
From a practical stand point though, many protanomalous and deuteranomalous people breeze through life with very little difficulty doing tasks that require normal color vision. Some may not even be aware that their color perception is in any way different from normal. The only problem they have is passing color vision tests.
Tomatoes, limes and oranges at a produce Market in Ecuador. Dichromat individuals see no perceptible difference between red, orange, yellow, and green. All these colors that seem so different to the normal viewer appear to be the same color for this two percent of the population.
People with Dichromacy
Dichromat individuals normally know they have a color vision problem and it can effect their lives on a daily basis. Dichromats are lacking one of the three cone types entirely. Dichromacy is much less common than anomalous trichromacy, but for protanopia and deutanopia at least, the effect is similar. (This is because in anomalous trichromacy the L-cone and M-cone have abnormally similar sensitivity curves, almost as though the two cone types have merged into one.)
Protanopia (one out of 100 males):
For the protanope, the brightness of red, orange, and yellow is much reduced compared to normal. This dimming can be so pronounced that reds may be confused with black or dark gray, and red traffic lights may appear to be extinguished. They may learn to distinguish reds from yellows and from greens primarily on the basis of their apparent brightness or lightness, not on any perceptible hue difference. Violet, lavender, and purple are indistinguishable from various shades of blue because their reddish components are so dimmed as to be invisible. Pink flowers, reflecting both red light and blue light, may appear just blue to the protanope.
Deuteranopia (one out of 100 males):
The deuteranope suffers the same hue discrimination problems as the protanope, but without the abnormal dimming. The names red, orange, yellow, and green really mean very little to him aside from being different names that every one else around him seems to be able to agree on. Similarly, violet, lavender, purple, and blue, seem to be too many names to use logically for hues that all look alike to him.
People with Achromatopsia and blue cone monochromacy
Achromatopsia is a form of colorblindness characterized by the total loss of all color vision. It is quite rare, but the effects are quite severe. Because an achromatope's vision comes entirely from the rods, it works best at low light levels, and at the periphery of view. Achromatopes must wear dark sunglasses in daylight or bright indoor conditions.
Blue cone monochromacy is a condition in which the L-cones and M-cones are missing, leaving just the S-cones and the rods. Because the S-cones do not contribute to our perception of brightness, blue cone monochromats have the same problems with bright lights as do achromatopes, although they are able to distinguish a small range of colors.
