Colorblindness
While recognizing that there are rarer subtypes of colorblindness, we are confining our discussion to the most common type: red-green colorblindness.
Most individuals with red-green colorblindness—about 80%—have an abnormal version of the red or green cone opsin. The remaining 20% entirely lack either the red or green cone opsin. In the cases of abnormal opsins, specific amino acid substitutions subtly alter the electrical charge in the retinal pocket. This changes the amount of photon energy needed to boost the pi electron to a higher orbital and release the retinal double bond. The result is to narrow the difference in wavelength sensitivity between the red and green cones.
In people with altered green opsins (deuteranomaly, or deutans), the green cone’s response overlaps too closely with the normal red cone. Conversely, in those with altered red opsins (protanomaly, or protans), the red cone’s response is too similar to the normal green cone. In both cases, the contrast between red and green cone activity becomes too small for reliable differentiation.
As a result, individuals with red-green colorblindness have difficulty distinguishing hues that differ only slightly in their red-green balance. However, most can still perceive colors with large red–green differences—such as bright reds or greens—relatively well.
Blue shades typically pose little difficulty because short wavelengths stimulate blue cones, but cause little activation of the red and green cones in the surround. Yellows are also usually perceived accurately: yellow light stimulates both red and green cones but very little blue. Because the blue–yellow contrast compares blue cone activity to the sum of red and green cone responses, this signal remains robust even when one of the red or green cones is defective. The summing process is not significantly affected by which cone (red or green) contributes more.
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The genes that code for the red and green cone opsins are located on the X chromosome. Since males have only one X chromosome, a defect in one of these genes will result in colorblindness. In contrast, females have two X chromosomes, so both copies must carry the defect for colorblindness to occur—making it significantly less common in women.
The red and green opsins are normally quite similar, differing only by about 20 of their 350 amino acids. When there are fewer amino acid differences in the opsin proteins, the shift in frequency response is usually smaller, and the colorblindness is milder. Exactly which amino acid substitutions are present is also important.
Another important but less commonly discussed factor in colorblindness is the significant variability in the ratio of red to green cones among individuals. Since cone activity is compared against the average of hundreds of surrounding cones, the relative abundance of red or green cones in that surround region can influence color perception—especially when the spectral difference between cone types is small. For example, a green cone surrounded by mostly red cones will produce a different contrast signal than if it were surrounded by mostly green cones.
This variability adds another layer of complexity to how colorblind individuals perceive the world.