Dr. rer. nat. Daniela Oesterle is a molecular biologist, human geneticist as well as a trained medical editor. As a freelance journalist, she writes texts on health topics for experts and lay people and edits scientific articles by doctors in German and English. For a renowned publishing house she is responsible for the publication of certified advanced training courses for physicians.
At Red-green weakness is a genetically caused visual impairment of the eye. Affected individuals have weaker vision of red or green, making it difficult to distinguish between the two colors. You perceive the world as less colorful than people with normal vision. Read more about red-green vision deficiency and why it should not be confused with red-green blindness here.
Red-green deficiency: Description
Red-green deficiency (anomalous trichromasia) belongs to the color sense disorders of the eye. Affected people recognize the colors red or green with different intensities and can distinguish between them poorly or not at all. Colloquially, the term "red-green blindness" is often used for this Red-green blindness used. However, this is not correct, because in red-green deficiency the vision for red and green is still present to a different extent. In true red-green blindness (a form of color blindness), however, those affected are actually blind to the corresponding color.
The term red-green deficiency includes two visual deficiencies:
- Red visual impairment (protanomaly)Affected persons see the color red more weakly and can distinguish it with difficulty from green.
- Green visual impairment (deuteranomaly)Affected persons perceive the color green more poorly and can hardly distinguish it from red.
Both visual impairments are genetic defects that affect the sensory cells for color vision.
sensory cells and color vision
Color vision is an extremely complex process with essentially three important manipulated variables: Light, sensory cells and brain.
Everything we see during the day reflects light of different wavelengths. This light hits three different light-sensing cells in the retina (retina or inner lining of the eye):
- Blue cone cells (B cone or S cone for "short"), i.e. short-wave light)
- Green cone cells (G cone or M cone for "medium"), so medium wave light)
- Red-cone cells (R-cone or L-cone for long), i.e. long-wave light)
They contain a pigment called Rhodopsin, Which is made up of the protein opsin and the smaller molecule 11-cis-retinal. However, opsin has a slightly different structure depending on the type of cone and is thus excited by different wavelengths of light – the basis for color vision: The opsin in the blue cones reacts particularly intensively to short-wave light (blue range), that of the green cones particularly to medium-wave light (green range), and that of the red cones mainly to long-wave light (red range).
Each cone cell thus covers a specific wavelength range, with overlapping ranges. The blue cones are most sensitive at a wavelength around 430 nanometers, the green cones at 535 nanometers and the red cones at 565 nanometers. This covers the whole color spectrum from red to orange, yellow, green, blue to violet back to red.
Millions of different shades
Now when light of appropriate wavelength hits the opsin of the B, G and R cones, the 11-cis retinal changes its chemical structure and activates a series of steps within the cell and eventually neighboring neurons. These in turn transmit the light impulses to the brain, where they are sorted, compared and interpreted.
Since the brain is capable of distinguishing about 200 color tones, approximately 26 saturation tones and circa 500 levels of brightness, people can several million shades except when a cone cell does not work properly, as in the case of red-green deficiency.
Red-green weakness: cone cells weaken
In the case of red-green weakness, the Opsin the green or the red cone is not fully functional. Reason is a chemical change in its structure:
- Red-vision: The opsin of the R cones is not most sensitive at 565 nanometers, but the maximum of its sensitivity has shifted towards green. The red cones therefore no longer cover the entire wavelength range for the color red and react more strongly to green light. The more the sensitivity maximum is shifted towards that of the green cones, the fewer red hues can be detected and the more difficult it is to distinguish red from green.
- Green vision deficiencyHere it is the other way round: the sensitivity maximum of the opsin of the G cones is shifted into the red wavelength range. Thus, fewer shades of green are perceived, and green can be more difficult to distinguish from red.
The red-green weakness is therefore not to be confused with the real red-green blindness, where the function of the red or green cones is completely lost. Red-green blind are completely blind to red or green.
Red-green deficiency: symptoms
Compared to people with normal vision, people with red-green deficiency perceive far fewer colors: although they have normal vision for various shades of blue and yellow, they see red and green less clearly. Red-green impairment always affects both eyes.
In which way affected persons still recognize the colors depends on the extent of the red-green deficiency: If the wavelength range of the R cones, for example, is only slightly shifted into that of the G cones, the affected person can see red and green relatively well, occasionally similarly well as a person with normal sight. However, the more the wavelength ranges of the G and R cones overlap, the less well affected people recognize the two colors: they are described in a wide variety of shades – from brownish-yellow to shades of gray.
Red-green weakness: causes and risk factors
The red-green weakness is genetically conditioned and therefore always innate:
The genetic defect is located on the gene for opsin of the green cones (in green vision deficiency) or on the opsin gene of the red cones (in red vision deficiency). The defect occurs during the first cell division of the fertilized egg, when paternal and maternal genetic material mixes. In this process ("crossover") called), the genes can be damaged in different ways. But in all cases they lose gene sequences. The expression of the red-green weakness depends on which gene areas are lost, because some areas are more important for the function or the sensitivity maximum than others.
Red-green deficiency affects more men than women
Both opsin genes are located on the X chromosome, which is why red-green deficiency occurs much more frequently in men than in women: men have only one X chromosome, whereas women have two. In case of a genetic defect of one of the opsin genes the male has no alternative, the female on the other hand can fall back on the intact gene of the second chromosome. If, however, the second gene is also defective, the red-green vision defect also shows up in the woman.
Figures prove that this is rarely the case: About 1.1 percent of men and 0.03 percent of women have red-vision deficiency. Green-vision impairment affects about five percent of men and 0.5 percent of women.
Red-green deficiency: examinations and diagnosis
In order to determine a red-green deficiency, the ophthalmologist will first talk to you in detail (anamnesis). For example, he may ask the following questions:
- Do you know someone in your family with a red-green deficiency??
- Do you only see blue and yellow as well as shades of brown or gray??
- Have you ever seen red or green?
- Do not see red and green with only one eye or are both eyes affected?
Color vision tests
In order to detect a red-green deficiency demonstrably, the ophthalmologist will ask you to look at so-called pseudoisochromatic plates such as the Ishihara boards to consider. These consist of many small circles representing numbers or figures. The background colors and the colors of the figures differ only in hue, but not in brightness and saturation. Therefore, only a healthy person with normal vision can see the figures, a person with red-green weakness cannot. This is how this color vision test works:
The panels are placed in front of your eyes at a distance of about 75 centimeters. Now the doctor asks you to look at the depicted figures or numbers with both eyes or only with one eye. numbers with both eyes or only with one eye. If you have a figure resp. If you don’t recognize the number within the first three seconds, the result is "wrong" or "uncertain". From the number of wrong or uncertain answers there are indications for a red-green disorder.
For children from the age of three the Color-Vision-Testing-Made-Easy-Test (CVTME-Test) is suitable. It does not show numbers or complicated figures, but simple symbols like circles, stars, squares or dogs.
Furthermore there are Color tests like the Farnsworth D15 test. Here you have to sort hats or chips of different colors.
Another way to diagnose a red-vision deficiency or green-vision deficiency is with a special device, the so-called Anomaloscope. Here the patient has to look through a tube at a halved circle. The halves of the circle are of different colors. With the help of turning wheels, the patient should now try to match the colors and their intensity:
A person with good vision can match both hue and intensity, while a person with poor vision can only match intensity. In addition, a red-sighted person will mix in too much red, and a green-sighted person will mix in too much green.
Read more about the tests
Find out here which examinations can be useful in this disease: