Tetrachromacy

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 and builds about one million distinguishable colours from them¹.

The genes for the long and medium cones sit on the X chromosome. Small variants in these genes shift a cone’s peak sensitivity. A man has one X chromosome, so a shifted gene replaces a normal one and he becomes colour blind. 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 types¹. About 12% of women carry such a combination¹.

Having 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. They see colour like everyone else¹.

In 2010, Gabriele Jordan and colleagues at Newcastle University tested 24 women who carried a fourth cone gene¹. One of them, known as cDa29, could tell apart colour mixtures that all trichromats see as identical. She is the first confirmed functional tetrachromat¹. An Australian artist, Concetta Antico, has since been studied and described as a tetrachromat². A true tetrachromat might in theory see about 100 million colours, but that figure is a calculation, not a measurement¹.

How common it is

The gene combination is uncommon in women and almost never present in men. Functional tetrachromacy, where the fourth cone actually changes what a person sees, has only been shown in a handful of people¹.

History and culture

Online quizzes that claim to detect tetrachromacy do not work. 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 laboratory¹. The claim that 25% of people are tetrachromats, which spread from a viral quiz, is wrong¹.