- Colour blindness, or colour vision deficiency, is a defect regarding the vision of an individual, hampering their ability to perceive colour
What Genes are Involved and Where are they Located?
- The genes that, as a result of a mutation, can cause colour blindness are OPN1LW, OPN1MW, and OPN1SW.
- The OPN1LW and OPN1MW genes are located on the X chromosome, which is one of the two sex chromosomes.
- Red-green color vision defects and blue colour vision defects are inherited in an X-linked recessive pattern.
- In males (who have only one X chromosome), one genetic change is sufficient to cause the condition.
- Males are affected by X-linked recessive disorders much more frequently than females because in females (who have two X chromosomes), a genetic change would have to occur on both copies of the chromosome to cause the disorder.
- A characteristic of this X-linked inheritance is that fathers cannot pass the X-linked trait to their sons.
What Type of Mutation Causes the Disorder?
- Mutations in the OPN1LW, OPN1MW, and OPN1SW genes are the cause of most forms of colour blindness
- The OPN1LW, OPN1MW, and OPN1SW genes provide instructions for making the three pigments (known as opsin) in cone cells.
- A point mutation, such as a missense, insertion or deletion mutation in any of the genes mentioned above can result in this vision defect
What is the Function of the Protein the Genes Code for?
- The proteins produced from genes OPN1LW, OPN1MW, and OPN1SW play essential roles in colour vision
- They are found in the retina, the light-sensitive tissue at the back of the eye.
- The retina contains two types of light receptor cells, called rods and cones, which transmit visual signals from the eye to the brain.
- Rods provide vision in low light whereas cones provide vision in bright light, including color vision.
- There are three types of cones, each containing a specific pigment protein, or opsin (red, blue and green) that is most sensitive to particular wavelengths of light. The brain combines input from all three types of cones to produce normal color vision.
- The pigment protein, or opsin, made from the OPN1LW gene is more sensitive to light in the yellow/orange part of the visible spectrum (long-wavelength light). They are known as L cones.
- The opsin made from the OPN1MW gene is more sensitive to light in the middle of the visible spectrum (yellow/green light). They are known as M cones.
- The opsin made from the OPN1SW gene is more sensitive to light in the blue/violet part of the visible spectrum (short-wavelength light), known as S cones.
What Effect do Mutations have on the Protein the Gene Codes for?
- Genetic changes involving the OPN1LW or OPN1MW gene cause red-green colour vision defects.
- A missense, insertion or deletion mutation in either of these genes will lead to an absence of L or M cone opsin (pigments) affecting red-green colour vision
- A nonsense mutation in either of these genes will result in the production of abnormal, incomplete opsin pigments in these cones that will also affect red-green color vision.
- Blue-yellow colour vision defects result from mutations in the OPN1SW gene.
- Missesense, insertion or deletion mutations lead to the premature destruction of S cone opsin.
- Nonsense mutations produce defective S cone opsin.
- Impaired S cone function alters perception of the colour blue, making it difficult or impossible to detect differences between shades of blue and green and causing problems with distinguishing dark blue from black.
What is the Mechanism of the Disorder?
- Mutations in any of the aforementioned genes results in a deficiency or complete lack of one or more of the essential color-sensitive cone cells located in the retina of the eye
- Working in tandem, these three pigments, or opsin, yield the range of colours an individual perceives in normal colour vision (a.k.a trichromatism)
- Problems involving the s cone are rare; they are much more common in those involving the l and m cone
What are the Symptoms of the Disorder?
- Colour blindness, or colour vision deficiency, is a defect of the vision of an individaul that hampers their ability to perceive colours
- Some individual are completely colour blind, meaning all colours are perceived as different shades of grey (ha ha). This is known as achromatopsia or monochromatism.
- People lacking functioning red cones (protanopia) or green cones (deuteranopia) confuse the colors red and green; the spectrum of color they see runs from yellow through white to blue.
- Individuals with all three types of cones, but faulty red or green cones tend to confuse red and green.
- Some people with colour blindness are unaware of it
Can the Disorder be Treated?
- Faulty colour vision is not only the result of a genetic mutation. It may also be a result of aging, eye trauma or exposure to certain chemicals or drugs.
- Specially tinted lenses exist that can enhance the ability of some people with red-green deficiency to distinguish colors by means of relative brightness.
- However, inherited colour blindness cannot be treated. Those who suffer from severe colour blindness should not partake in tasks that require colour distinction
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"Color Blindness." Funk & Wagnalls New World Encyclopedia (2014): 1p. 1. Funk & Wagnalls New World Encyclopedia. Web. 15 May 2015.
"Color Vision Deficiency." Genetics Home Reference. Web. 15 May 2015.
Simunovic, M. P. "Colour Vision Deficiency." Eye 24.5 (2010): 747. Advanced Placement Source. Web. 15 May 2015.