Tetrachromacy
Also called: Four-cone colour vision · Functional tetrachromacy
Most people have three kinds of colour-sensing cone. About 12% of women carry a fourth. In theory they can tell apart colours that look identical to everyone else. Clear cases are extremely rare, and online tests cannot detect it.
How common
Does this happen to you?
How it works
Colour vision depends on cone cells in the retina. Most people have three types, tuned to long, medium, and short wavelengths. The brain compares their signals. One researcher estimates that this gives about one million colours4.
The genes for the long and medium cones sit on the X chromosome. Small variants shift a cone’s peak sensitivity. A man has one X chromosome, so a shifted gene replaces a normal one and he becomes colour blind2. A woman has two X chromosomes. If one carries a normal gene and the other carries a shifted gene, she can end up with four cone types3. Some 12% of women carry such a combination3.
A fourth cone type does not guarantee a fourth colour channel. The brain has to wire the extra cone into its colour comparisons. In most carriers it appears not to, and they see colour like everyone else3.
In 2010, Gabriele Jordan and colleagues tested 24 women known to carry the gene for deuteranomaly, a mild colour blindness3. One of them, known as cDa29, could tell apart colour mixtures that all trichromats see as identical. She is the best-known candidate for functional tetrachromacy, but her case is not fully proven13. A true tetrachromat might in theory see about 100 million colours, but that figure is a calculation, not a measurement4.
How common it is
Two different numbers get mixed up here. The first is a carrier share. Some 12% of women carry the gene pattern for a fourth cone type3. That is a share of women, not of everyone. Women are about half of all people, so it works out near 6% of everybody, and men almost never carry it3. The second number is the count of people who pass lab tests for using the extra cone. Jordan and colleagues tested 24 carriers and found 13. Nobody has screened a large random sample, so the share of carriers who really see extra colours is unknown.
Is it a problem?
No. No health problem is known to come with carrying a fourth cone type. It is not a disorder. In a man, the same shifted gene causes colour blindness instead2. The opposite question comes up more often: people wonder if they might be a tetrachromat after taking an online quiz. No screen-based quiz can test for it.
History and culture
Online quizzes cannot detect tetrachromacy. A normal screen produces every colour from three primaries, so it cannot show a colour that only a fourth cone can detect. Real testing uses precise mixtures of single-wavelength lights in a laboratory3.
Sources and further material
- vision.psychol.cam.ac.uk The dimensionality of color vision in carriers of anomalous trichromacy Says some 12% of women are carriers. Tested 24 carriers and found 1 with tetrachromatic vision.
- web.archive.org Some women may see 100 million colors, thanks to their genes Jay Neitz estimates about 1 million colours for a normal person and, in theory, 100 million for a true tetrachromat.
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Supertasters
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