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|Color Vision Deficiencies|
Color vision deficiencies (color blindness) are a group of conditions that affect the perception of color. They cause a range of changes in color vision, from mild difficulty with distinguishing shades to a total inability to detect color.
Color vision deficiencies are divided into three major categories: red-green color vision defects, blue-yellow color vision defects, and a complete absence of color vision.
Red-green color vision defects are the most common form of color vision deficiency. Affected individuals have trouble distinguishing between shades of red and green. They see these colors differently than most people and may have trouble naming different hues. This condition affects males more often than females. Among populations with Northern European ancestry, it occurs in about 8 percent of males and 0.5 percent of females. Red-green color vision defects have a lower incidence in almost all other populations studied.
Blue-yellow color vision defects, which are rarer, cause problems with differentiating shades of blue and yellow. They affect males and females equally. This condition occurs in fewer than 1 in 10,000 people worldwide.
These two forms of color vision deficiency disrupt color perception but do not affect the sharpness of vision (visual acuity).
Complete absence of color vision, called achromatopsia, is uncommon. People with complete achromatopsia cannot perceive any colors. They see only black, white, and shades of gray. A milder form of this condition, incomplete achromatopsia, may allow some color discrimination. People with achromatopsia almost always have additional problems with vision including reduced visual acuity, increased sensitivity to light (photophobia), and small involuntary eye movements called nystagmus. This condition affects an estimated 1 in 30,000 people.
The retina, a light-sensitive tissue at the back of the eye, contains two types of light receptor cells called rods and cones. These cells transmit visual signals from the eye to the brain. Rods are responsible for vision in low light. Cones provide vision in bright light, including color vision. Three types of cones each contain a special pigment (a photopigment) that is most sensitive to a particular wavelength of light. The brain combines input from all three types of cones to produce normal color vision.
Specific genes provide instructions for making the three photopigments. The OPN1LW gene makes a pigment that is more sensitive to light at the red end of the visible spectrum, and cones with this pigment are sometimes called long-wavelength-sensitive or L cones. The OPN1MW gene makes a pigment that is more sensitive to light in the middle of the visible spectrum (yellow/green light), and cones with this pigment are often called middle-wavelength-sensitive or M cones. The OPN1SW gene makes a pigment that is more sensitive to light at the blue/violet end of the visible spectrum, and cones with this pigment are usually called short-wavelength-sensitive or S cones.
Genetic changes involving the OPN1LW and OPN1MW genes cause red-green color vision defects. These changes lead to an absence of L or M cones or the production of cones with abnormal visual properties that affect red-green color vision. Blue-yellow color vision defects result from mutations in the OPN1SW gene. These mutations inactivate the short-wave-sensitive pigment, which probably leads to the premature destruction of S cones or the production of defective cones. A loss of S cones impairs perception of the color blue and makes it difficult or impossible to detect differences between shades of blue and green.
Changes in the CNGA3, CNGB3, and GNAT2 genes are responsible for achromatopsia. Each of these genes provides instructions for making a protein that is involved in the normal function of cones in the retina. Mutations in any of these genes prevent all three types of cones from reacting appropriately to light. As a result, most people with mutations in one of these genes must depend on rods alone for vision. They typically have no color vision and often have other visual problems as well. Some people with mutations in CNGA3 have incomplete achromatopsia, which may allow some cone function and limited color vision.
A particular form of incomplete achromatopsia, called blue cone monochromacy, occurs when genetic changes prevent both L and M cones from functioning normally. People with this condition have only S cones. Because the brain must compare input from at least two types of cones to detect color, people who have only functional S cones have very poor color vision.
Some problems with color vision are not caused by gene mutations. These nonhereditary conditions, which are described as acquired color vision deficiencies, occur in people with other eye disorders. Specifically, acquired color vision deficiences can result from diseases involving the retina, the nerve that carries visual information from the eye to the brain (the optic nerve), or areas of the brain involved in processing visual information.
How do people inherit color vision deficiency?
The types of color vision deficiency have different patterns of inheritance. Red-green color vision defects and blue cone monochromacy are inherited in an X-linked recessive pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation must be present in both copies of the gene to cause the disorder. Males are affected by X-linked recessive disorders much more frequently than females. A striking characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.
Blue-yellow color vision defects are inherited in an autosomal dominant pattern, which means one copy of the altered OPN1SW gene in each cell is sufficient to cause the condition.
Complete achromatopsia is inherited in an autosomal recessive pattern, which means two copies of the CNGA3, CNGB3, or GNAT2 gene in each cell are altered. Most often, the parents of an individual with an autosomal recessive disorder are carriers of one copy of the altered gene but do not show signs and symptoms of the condition.
There is no cure for color vision deficiencies. However, certain types of tinted contact lenses may help affected individuals distinguish different colors better.