Color blindness, also known as color vision deficiency, is the decreased ability to see color or differences in color. It is most often an inherited genetic disorder, but can also be acquired later in life due to disease or injury to the eye, optic nerve, or parts of the brain.
What causes color blindness?
The most common cause of inherited color blindness is a defect in the genes that code for the retinal cone cells that detect color. There are three types of retinal cone cells, each detecting a different range of light wavelengths that we see as red, green or blue light. If any of these cone cells are absent, defective or detect a different wavelength of light, color vision will be impaired.
The genes for the red and green retinal cone cells are located on the X chromosome. If a genetic mutation occurs on one of these genes, it will cause color blindness. Since males only have one X chromosome, they are more likely to express color blindness if they inherit an abnormal X chromosome from their mother. Females have two X chromosomes, so a defect in one X chromosome may be compensated for by the other normal X chromosome. For this reason, inherited color blindness affects males much more often than females.
Types of color blindness
There are several types of inherited color blindness depending on which photopigments in the retinal cone cells are affected. The main types are:
- Red-green color blindness – the most common type where there is reduced sensitivity to red or green light. This makes it difficult to distinguish between reds, greens, browns and oranges.
- Blue-yellow color blindness – a rare type affecting blue and yellow shades. This makes blue appear greener and yellows more pink.
- Complete color blindness (achromatopsia) – very rare, where no color is seen at all, only shades of grey.
Sometimes color blindness only affects certain hues or shades within the red-green or blue-yellow spectrum. There are also milder forms where color vision is impaired rather than completely absent.
Is color blindness sex-linked to the X chromosome?
Yes, the most common types of inherited color blindness are sex-linked conditions that primarily affect males. The genes for the long wavelength (red) and medium wavelength (green) photopigments are located on the X chromosome at Xq28. Defects in these two genes account for most cases of red-green color blindness.
Since males only have one X chromosome, they only need one defective gene to cause color blindness. Females have two X chromosomes, so usually one normal gene can compensate if the other is defective. For this reason, red-green color blindness affects around 1 in 12 males but only 1 in 200 females of European descent.
| Type of color blindness | Affected photopigments | Location of genes |
|---|---|---|
| Red-green | Long and medium wavelength sensitive cones | X chromosome |
| Blue-yellow | Short wavelength sensitive cones | Chromosome 7 |
| Complete achromatopsia | All cone cells | Chromosomes X, 2, 11 |
The table summarizes the common types of inherited color blindness and the chromosome locations of the responsible genes. This shows that the X-linked red-green color blindness is the most prevalent type affecting males.
Genetics of red-green color blindness
The red and green photopigment genes on the X chromosome are arranged in a head-to-tail tandem array. There can be varying numbers of these gene copies between different individuals. Normal color vision requires at least one long wavelength (red) and one medium wavelength (green) gene.
There are several genetic mutations that can disrupt color vision:
- Missense mutation – Single nucleotide change that alters an amino acid in the photopigment protein, changing its color sensitivity.
- Gene deletion – Partial or complete deletion of the red or green pigment gene.
- Gene rearrangement – The genes can abnormally recombine, sometimes creating fusion genes with altered color sensitivity.
- Nonsense mutation – Premature stop codon leads to incomplete, nonfunctional photopigment protein.
These genetic changes affect the peak sensitivity of the cone cells, which alters the wavelengths of light they detect. If the red and green sensitive cones detect similar wavelengths, it becomes difficult to distinguish red and green hues.
Is color blindness ever on the Y chromosome?
No, there are no genes on the Y chromosome involved in color vision. The Y chromosome carries few genes compared to the X chromosome, and mainly contains genes involved in male sexual development.
While the main causes of inherited color blindness are X-linked, the Y chromosome itself does not determine normal color vision in males. Rarely, color blindness can be caused by genetic mutations on the autosomes (non-sex chromosomes) such as chromosomes 2, 3, 7 or 11. But in most cases it is caused by abnormal red or green photopigment genes on the X chromosome.
Can color blindness be acquired later in life?
Yes, color vision defects can sometimes be acquired later in life rather than inherited. Causes include:
- Diseases affecting the retina such as macular degeneration and diabetes.
- Damage to the eye, optic nerve or visual cortex in the brain.
- Cataract surgery may affect color perception.
- Some medications like digoxin and hydroxychloroquine can cause color blindness.
- Aging causes yellowing of the lens and can impair blue color perception.
Unlike inherited color blindness which usually affects red-green perception, acquired color defects often impair blue-yellow color discrimination. The underlying mechanism is damage to the retinal cones or visual pathways, rather than genetic mutations affecting cone cell function.
Diagnosing color blindness
Color blindness is usually first detected in childhood when tested at school. Simple color vision tests use colored plates to screen for the ability to detect numbers or patterns among different hues. More specialized tests include:
- Ishihara test – Read colored numbers on plates
- Farnsworth D-15 test – Arrange color caps in hue order
- Hardy-Rand-Rittler plates – Match colored plates
- Lantern tests – Identify colored lights in the dark
These tests help determine the type and severity of color blindness. They can distinguish between inherited color defects versus acquired vision problems. Diagnosing color blindness is important for occupations requiring accurate color perception, such as pilots and electricians.
Is there any treatment for color blindness?
Currently there is no cure for inherited color blindness. The genetic defects in the cone photopigment genes that cause it cannot be corrected. However, some adaptive technologies can help the color blind to distinguish problematic hues:
- Special tinted lenses and filters that block overlapping wavelengths can improve color discrimination.
- Digital image processing can filter colors to appear more distinct on digital screens.
- Colorimetric devices can convert colors into melodies or patterns, providing audio/tactile information.
- Apps for smartphones can identify colors from the camera and provide a spoken name.
While these tools can assist with some daily challenges of color blindness, they do not restore normal color vision. With further research on gene therapies, there is hope of one day curing inherited color blindness.
Conclusion
In summary, the common red-green color blindness is an X-linked genetic disorder, with the photopigment genes located on the X chromosome. While very rare forms can involve other chromosomes, there are no genes on the Y chromosome that determine normal color vision. Color blindness affects a significant portion of males due to their single X chromosome. Better awareness and adaptive technologies help the color blind adapt, but a cure will rely on advances in gene therapy.
