Most of us share a common color vision sensory experience. Some people, however, have a color vision deficiency, which means their perception of colors is different from what most of us see. The most severe forms of these deficiencies are referred to as color blindness. People with color blindness aren’t aware of differences among colors that are obvious to the rest of us. People who don’t have the more severe types of color blindness may not even be aware of their condition unless they’re tested in a clinic or laboratory.
Inherited color blindness is caused by abnormal photopigments. These color-detecting molecules are located in cone-shaped cells within the retina, called cone cells. In humans, several genes are needed for the body to make photopigments, and defects in these genes can lead to color blindness.
There are three main kinds of color blindness, based on photopigment defects in the three different kinds of cones that respond to blue, green, and red light. Red-green color blindness is the most common, followed by blue-yellow color blindness. A complete absence of color vision —total color blindness – is rare.
Sometimes color blindness can be caused by physical or chemical damage to the eye, the optic nerve, or parts of the brain that process color information. Color vision can also decline with age, most often because of cataract – a clouding and yellowing of the eye’s lens.
Who gets color blindness?
As many as 8 percent of men and 0.5 percent of women with Northern European ancestry have the common form of red-green color blindness.
Men are much more likely to be colorblind than women because the genes responsible for the most common, inherited color blindness are on the X chromosome. Males only have one X chromosome, while females have two X chromosomes. In females, a functional gene on only one of the X chromosomes is enough to compensate for the loss on the other. This kind of inheritance pattern is called X-linked, and primarily affects males. Inherited color blindness can be present at birth, begin in childhood, or not appear until the adult years.
How Genes are Inherited
Genes are bundled together on structures called chromosomes. One copy of each chromosome is passed by a parent at conception through egg and sperm cells. The X and Y chromosomes, known as sex chromosomes, determine whether a person is born female (XX) or male (XY) and also carries other traits not related to gender.
In X-linked inheritance, the mother carries the mutated gene on one of her X chromosomes and will pass on the mutated gene to 50 percent of her children. Because females have two X chromosomes, the effect of a mutation on one X chromosome is offset by the normal gene on the other X chromosome. In this case, the mother will not have the disease, but she can pass on the mutated gene and so is called a carrier. If a mother is a carrier of an X-linked disease (and the father is not affected), there is a:
- 1 in 2 chance that a son will have the disease,
- 1 in 2 chance that a daughter will be a carrier of the disease,
- No chance that a daughter will have the disease.
In autosomal recessive inheritance, it takes two copies of the mutant gene to give rise to the disease. An individual who has one copy of a recessive gene mutation is known as a carrier. When two carriers have a child, there is a:
- 1 in 4 chance of having a child with the disease,
- 1 in 2 chance of having a child who is a carrier,
- 1 in 4 chance of having a child who neither has the disease nor is a carrier.
In autosomal dominant inheritance, it takes just one copy of the mutant gene to bring about the disease. When an affected parent with one dominant gene mutation has a child, there is a 1 in 2 chance that a child will inherit the disease.
How do we see color?
What color is a strawberry? Most of us would say red, but do we all see the same red? Color vision depends on our eyes and brain working together to perceive different properties of light.
We see the natural and artificial light that illuminates our world as white, although it is actually a mixture of colors that, perceived on their own, would span the visual spectrum from deep blue to deep red. You can see this when rain separates sunlight into a rainbow or a glass prism separates white light into a multi-color band. The color of light is determined by its wavelength. Longer wavelength corresponds to red light and shorter wavelength corresponds to blue light.
Strawberries and other objects reflect some wavelengths of light and absorb others. The reflected light we perceive as color. So, a strawberry is red because its surface is only reflecting the long wavelengths we see as red and absorbing the others. An object appears white when it reflects all wavelengths and black when it absorbs all wavelengths.
Vision begins when light enters the eye and the cornea and lens focus it onto the retina, a thin layer of tissue at the back of the eye that contains millions of light-sensitive cells called photoreceptors. Some photoreceptors are shaped like rods and some are shaped like cones. In each eye there are many more rods than cones – approximately 120 million rods compared to only 6 million cones. Rods and cones both contain photopigment molecules that undergo a chemical change when they absorb light. This chemical change acts like an on-switch, triggering electrical signals that are then passed from the retina to the visual parts of the brain.
Rods and cones are different in how they respond to light. Rods are more responsive to dim light, which makes them useful for night vision. Cones are more responsive to bright light, such as in the daytime when light is plentiful.
Another important difference is that all rods contain only one photopigment, while cones contain one of three different photopigments. This makes cones sensitive to long (red), medium (green), or short (blue) wavelengths of light. The presence of three types of photopigments, each sensitive to a different part of the visual spectrum, is what gives us our rich color vision.
Humans are unusual among mammals for our trichromatic vision – named for the three different types of photopigments we have. Most mammals, including dogs, have just two photopigment types. Other creatures, such as butterflies, have more than three. They may be able to see colors we can only imagine.
Most of us have a full set of the three different cone photopigments and so we share a very similar color vision experience, but because the human eye and brain together translate light into color, each of us sees colors differently. The differences may be slight. Your blue may be more blue than someone else’s, or in the case of color blindness, your red and green may be someone else’s brown.
What are the different types of color blindness?
The most common types of color blindness are inherited. They are the result of defects in the genes that contain the instructions for making the photopigments found in cones. Some defects alter the photopigment’s sensitivity to color, for example, it might be slightly more sensitive to deeper red and less sensitive to green. Other defects can result in the total loss of a photopigment. Depending on the type of defect and the cone that is affected problems can arise with red, green, or blue color vision.
Red-Green Color Blindness
The most common types of hereditary color blindness are due to the loss or limited function of red cone (known as protan) or green cone (deutran) photopigments. This kind of color blindness is commonly referred to as red-green color blindness.
- Protanomaly: In males with protanomaly, the red cone photopigment is abnormal. Red, orange, and yellow appear greener and colors are not as bright. This condition is mild and doesn’t usually interfere with daily living. Protanomaly is an X-linked disorder estimated to affect 1 percent of males.
- Protanopia: In males with protanopia, there are no working red cone cells. Red appears as black. Certain shades of orange, yellow, and green all appear as yellow. Protanopia is an X-linked disorder that is estimated to affect 1 percent of males.
- Deuteranomaly: In males with deuteranomaly, the green cone photopigment is abnormal. Yellow and green appear redder and it is difficult to tell violet from blue. This condition is mild and doesn’t interfere with daily living. Deuteranomaly is the most common form of color blindness and is an X-linked disorder affecting 5 percent of males.
- Deuteranopia: In males with deuteranopia, there are no working green cone cells. They tend to see reds as brownish-yellow and greens as beige. Deuteranopia is an X-linked disorder that affects about 1 percent of males.
Blue-Yellow Color Blindness
Blue-yellow color blindness is rarer than red-green color blindness. Blue-cone (tritan) photopigments are either missing or have limited function.
- Tritanomaly: People with tritanomaly have functionally limited blue cone cells. Blue appears greener and it can be difficult to tell yellow and red from pink. Tritanomaly is extremely rare. It is an autosomal dominant disorder affecting males and females equally.
- Tritanopia: People with tritanopia, also known as blue-yellow color blindness, lack blue cone cells. Blue appears green and yellow appears violet or light grey. Tritanopia is an extremely rare autosomal recessive disorder affecting males and females equally.
Complete color blindness
People with complete color blindness (monochromacy) don’t experience color at all and the clearness of their vision (visual acuity) may also be affected.
There are two types of monochromacy:
- Cone monochromacy: This rare form of color blindness results from a failure of two of the three cone cell photopigments to work. There is red cone monochromacy, green cone monochromacy, and blue cone monochromacy. People with cone monochromacy have trouble distinguishing colors because the brain needs to compare the signals from different types of cones in order to see color. When only one type of cone works, this comparison isn’t possible. People with blue cone monochromacy may also have reduced visual acuity, near-sightedness, and uncontrollable eye movements, a condition known as nystagmus. Cone monochromacy is an autosomal recessive disorder.
- Rod monochromacy or achromatopsia: This type of monochromacy is rare and is the most severe form of color blindness. It is present at birth. None of the cone cells have functional photopigments. Lacking all cone vision, people with rod monochromacy see the world in black, white, and gray. And since rods respond to dim light, people with rod monochromacy tend to be photophobic – very uncomfortable in bright environments. They also experience nystagmus. Rod monochromacy is an autosomal recessive disorder.
How is color blindness diagnosed?
Eye care professionals use a variety of tests to diagnose color blindness. These tests can quickly diagnose specific types of color blindness.
The Ishihara Color Test is the most common test for red-green color blindness. The test consists of a series of colored circles, called Ishihara plates, each of which contains a collection of dots in different colors and sizes. Within the circle are dots that form a shape clearly visible to those with normal color vision, but invisible or difficult to see for those with red-green color blindness.
The newer Cambridge Color Test uses a visual array similar to the Ishihara plates, except displayed on a computer monitor. The goal is to identify a C shape that is different in color from the background. The “C” is presented randomly in one of four orientations. When test-takers see the “C,” they are asked to press one of four keys that correspond to the orientation.
The anomaloscope uses a test in which two different light sources have to be matched in color. Looking through the eyepiece, the viewer sees a circle. The upper half is a yellow light that can be adjusted in brightness. The lower half is a combination of red and green lights that can be mixed in variable proportions. The viewer uses one knob to adjust the brightness of the top half, and another to adjust the color of the lower half. The goal is to make the upper and lower halves the same brightness and color.
The HRR Pseudoisochromatic Color Test is another red-green color blindness test that uses color plates to test for color blindness.
The Farnsworth-Munsell 100 Hue Test uses a set of blocks or pegs that are roughly the same color but in different hues (shades of the color). The goal is to arrange them in a line in order of hue. This test measures the ability to discriminate subtle color changes. It is used by industries that depend on the accurate color perception of its employees, such as graphic design, photography, and food quality inspection.
The Farnsworth Lantern Test is used by the U.S. military to determine the severity of color blindness. Those with mild forms pass the test and are allowed to serve in the armed forces.
Are there treatments for color blindness?
There is no cure for color blindness. However, people with red-green color blindness may be able to use a special set of lenses to help them perceive colors more accurately. These lenses can only be used outdoors under bright lighting conditions. Visual aids have also been developed to help people cope with color blindness. There are iPhone and iPad apps, for example, that help people with color blindness discriminate among colors. Some of these apps allow users to snap a photo and tap it anywhere on the image to see the color of that area. More sophisticated apps allow users to find out both color and shades of color. These kinds of apps can be helpful in selecting ripe fruits such as bananas, or finding complementary colors when picking out clothing.
How does color blindness affect daily life?
Color blindness can make it difficult to read color-coded information such as bar graphs and pie charts. This can be particularly troubling for children who aren’t yet diagnosed with color blindness since educational materials are often color-coded. Children with red-green color blindness may also have difficulty reading a green chalkboard when yellow chalk is used. Art classes, which require selecting appropriate colors of paint or crayons, may be challenging.
Color blindness can go undetected for some time since children will often try to hide their disorder. It’s important to have children tested, particularly boys, if there is a family history of color blindness. Many school systems offer vision screening tests that include color blindness testing. Once a child is diagnosed, he or she can learn to ask for help with tasks that require color recognition.
Simple everyday tasks like cooking meat to the desired color or selecting ripe produce can be a challenge for adults. Children might find food without bright color as less appetizing. Traffic lights pose challenges since they have to be read by the position of the light. Since most lights are vertical, with green on bottom and red on top, if a light is positioned horizontally, a color blind person has to do a quick mental rotation to read it. Reading maps or buying clothes that match colors can also be difficult. However, these are relatively minor inconveniences and most people with color blindness learn to adapt.
What research is being done?
NEI-supported researchers have used gene therapy to cure color blindness in adult monkeys. While red-green color blindness affects about 8 percent of Northern European-descended men, it affects all adult male squirrel monkeys because males of the species carry either the gene that makes red photopigment or the gene that makes green photopigment, but never both. The researchers injected the red photopigment gene into the retinas of male monkeys born without it. The gene was targeted to green cones and allowed those cells to respond to red light. The monkeys were able to see with full three-color (trichromatic) vision. This shows that even though the monkeys’ red cones had been absent from birth, the brain circuitry for detecting red was still in place—offering hope that a similar approach could help people who’ve been colorblind since birth.
In another study, NEI-supported researchers were able to restore some color perception in an animal model of rod monochromacy (in which all three cone types are missing), using a gene therapy approach in younger animals. The therapy combined gene delivery with the addition of neurotrophic factors – molecules that are known to help nerve cells grow. Further studies will be testing whether the therapy could be safe and effective in humans.
An ongoing NEI clinical trial is testing whether treatment with a growth factor alone could be enough to improve or restore the visual function of cone cells in people. This has the potential to help people with color blindness, as well as diseases that are the result of the loss of cones or cone function.
Researchers supported by NEI are also studying how cones develop in the retina and how they are maintained and preserved throughout the lifespan. This research could lead to therapies for color-blindness that occurs during childhood or later in life due to the gradual loss of cones.
Source: National Eye Institute: nei.nih.gov