The Genetics of Vision
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Many different errors in this gene can lead to choroideremia.
Paul Freund and Dr. Ian MacDonald, both from the University of Alberta, presented their studies of this genetic variation. The team identified several new, never-before-observed mutations in this gene. Choroideremia can be a very variable disease, with some men experiencing severe vision loss in young adulthood and others maintaining good vision into late middle age.
As well, the exhaustive evaluation of choroideremia patients in this study gives caregivers more information about the natural course of choroideremia so that they can better advise their patients. The milder forms of red-green color blindness are the anomalous trichromacies. There are 2 types—protanomaly and deuteranomaly—that parallel the 2 dichromatic types—protanopia and deuteranopia. Protanomalous trichromats are missing normal L photopigment just like the protanopes.
However, as the term for their condition implies, they have trichromatic color vision. Their trichromacy is not based on L, M, and S pigments like in those with normal color vision.
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In the classic descriptions of this disorder, protanomalous trichromats were said to have normal S blue and M green pigment but an abnormal or "anomalous" L red pigment. From what we know, the concept of an anomalous L pigment in protanomaly no longer seems apt. The molecular genetics results indicate that protanomalous trichromats can be more accurately characterized as having lost all L pigments. They have an S pigment remaining and 2 M or M-like pigments that are usually conceived as differing by a small shift in spectral peak.
The genetic basis for having 2 different M pigment genes is believed to arise from rearrangements within the normal tandem array of pigment genes. Most red-green color vision defects are believed to arise from gene rearrangements and, as we have explained, for most protan observers this "rearrangement" includes the deletion of all genes that could encode pigments falling into the normal L class. The rearrangements also create hybrid or "chimeric" genes in which some of the L gene sequences have been replaced by M gene sequences. These chimeric genes encode pigments with spectral properties that place them in the M class.
However, for all protanomalous trichromats, there is more than one pigment gene left remaining in the X chromosome array, which includes one or more typical M pigment genes. We use the term chimeric genes to refer to the variant forms of the human L and M pigment genes. There is an extremely high degree of genetic polymorphism in the human L and M genes, in people with normal and with defective color vision.
This variation can be explained as having arisen from shuffling of the L and M gene segments that has occurred in the process of human evolution. Thus, the genes in people with normal and defective color vision can be "chimeras" with different segments derived from what, in our early evolution, may have been original L and M genes.
In light of our current understanding, this term, chimera , seems preferable to the alternatives, hybrid or fusion genes, that were the terms introduced by Nathans et al 1 to describe an early molecular-genetic concept of the genes underlying color anomaly. A replacement of the term fusion gene with chimeric can serve to reinforce a shift in understanding.
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From the early studies, it was assumed that all normal color vision was based on a stereotyped normal L pigment gene and a stereotyped M pigment gene. Hybrid genes were conceived of as arising from isolated gene rearrangement events that caused color vision defects.
Evidence has accumulated during the past decade suggesting a history of extensive recombination between M and L genes. The other extensive variations in the pigment genes contribute to individual differences in normal color vision. Historically, there has been debate about the relationship between genes and pigments in people with normal color vision compared with those with color vision defects. The realization that there is widespread variation in the sequences of the M and L genes in normal color vision allows for the possibility of considerable overlap between the variant M genes in normal color vision and the subtypes of M genes in the color defect protanomaly.
However, there do appear to be differences in the distribution of variant M genes in those with normal color vision vs those with protanomaly such that some specific types of chimeric genes that occur in protanomalous trichromats have not been found in individuals with normal color vision. The most common type of anomalous trichromacy is deuteranomaly.
In fact, it is the most common of all inherited color vision defects by a large margin. In the United States, it is estimated to affect 1 in 20 men. It is also estimated to affect many more women than any of the other red-green color vision deficiencies; about 3 in women are deuteranomalous, a rate about 25 times higher than that of any other color vision defect. Like other forms of normal and anomalous trichromacy, deuteranomaly is based on 3 pigments. It is based on the presence of the S cones plus 2 spectral subtypes of L cones. As a basis for 2 spectral types of L pigments, all persons with deuteranomaly have at least 2 different genes to encode L pigments.
The genetics of normal and defective color vision.
In this aspect, the correspondence between genotype and phenotype is perfect and thus can be used reliably in the genetic diagnosis of deuteranomaly. In contrast, there is an aspect of deuteranomaly in which the relationship between genotype and phenotype is not clear at all, ie, most persons with deuteranomaly have M pigment genes and thus it is not understood why they do not have M cone function.
This is probably the most important unanswered question concerning the molecular genetics of color vision defects. There is evidence that persons with deuteranomaly lack an M cone contribution to vision because they lack both functional M cones and expressed M photopigment. However, what causes this loss is uncertain. Several hypotheses have been forwarded to explain the loss of M function. One hypothesis is that only the first 2 pigment genes in the X chromosome array are expressed and that the second L pigment gene that occurs in all persons with deuteranomaly displaces the M pigment gene out of an "expressed" position.
In summary, the red-green anomalous trichromacy can be explained as arising from the loss of one class of cone photopigment. Protanomalous trichromats lack pigments from the L class and persons with deuteranomaly lack pigments from the M class.
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In persons with protanomaly, L pigment genes are almost always missing. In about two thirds of deuteranomalous men, M genes are present and the reason for loss of function is not clear. Persons with protanomaly must always have at least 2 different M genes to encode 2 spectral subtypes of M pigment. One of those M genes can be a chimeric gene that is unlike the genes typically found in persons with normal color vision. Persons with deuteranomaly must have at least 2 different L genes to encode 2 spectral subtypes of L pigment.
These characteristics allow one to design a genetic test that would, most of the time, distinguish between the 2 classes of anomalous trichromacy, distinguish the anomaly from the corresponding dichromacy, and distinguish anomaly from normal color vision. It is a common misconception that red-green color blindness does not affect performance in daily tasks. Common everyday complaints among color-blind subjects include having difficulty navigating the highly color-coded World Wide Web on the computer Internet, seeing that someone has become sunburned or has a slight rash, reading color-coded maps, distinguishing traffic signals, and dressing in appropriately matching clothes.
The difference between normal color vision and dichromacy is large. The term dichromacy means, literally, 2 hues and derives from the fact that dichromats can match any color using mixtures of just 2 "primary" hues. However, the term dichromat is also appropriate because dichromats see only 2 hues. To them, objects are black, white, shades of gray, or 1 of 2 hues. In contrast, people with normal color vision see more than different hues in addition to black, white, and gray.
Dichromats confuse red with green, and they confuse, with red and green, all colors in the spectrum that fall between them, including yellow, orange, and brown. They see blue and violet as the same color, and blue-green is indistinguishable from white or gray.
Magenta and its pastel counterpart pink also appear white or gray. The most severely affected anomalous trichromats have color vision that is similar to that of a dichromat. Mildly affected anomalous trichromats have more difficulty distinguishing between pastel colors than between the saturated versions of those same colors.
They may see the difference between red and green, but cannot see the difference between more similar colors such as olive green and brown. It is often said that the term color blind is a misnomer. However, it is difficult to find a more appropriate term for individuals who are unable to distinguish all but 2 of the more than hues that are normally seen as different. Dichromats may say that they see many colors but have difficulty with certain shades. In truth, dichromats become adept at using brightness and saturation differences as visual cues, and they learn to call these differences "colors.
The 2 most devastating problems that can be encountered by color-blind people involve 1 career choices and 2 early education. For example, all too often it happens that a young man has planned his whole life to be a police officer only to find out at the age of 25 years, after years of hard work, that he can never attain his life's dream because of color blindness.
There are many similar stories about people prevented from attending the Air Force Academy, being airline pilots, and entering other professions that require normal color vision. Great frustration is experienced by individuals trying to enter a field that has no formal color vision requirements, yet good color vision is required for success, as, for example, is true in chemistry and geology. For young children, problems may arise at school because color blindness causes a form of visual miscommunication.
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Children live in a world of natural and human-made color coding. In the early grades, colors are used as tools of communication. Children are expected to learn to differentiate colors, know color names, and associate colors with specific meanings in their lives.