We perceive light and color because of photoreceptor cells in our retina called rods and cones. Rods help us see low contrast images, motion, and during nighttime. Cones help us see high contrast sharp images and colors. Humans with normal color vision have three types of cones: s, m, and l cones. These cones help us see colors of different wavelengths, i.e., from lower wavelength color like blue to a higher wavelength color like red. Hence, a person with normal color vision is a trichromat, tri meaning three, chroma meaning colors. Trichromats can differentiate between about 1- 2 million colors. However, about 12% of women have a special 4th cone that enables them to see in more detail and can differentiate between 100 million colors. This condition is called tetrachromacy, tetra meaning four, chroma meaning colors. Women with tetrachromacy are called tetrachromats.
Tetrachromacy is usually caused by a genetic mutation in the X-linked chromosome and is predominantly found in women. If you recollect middle school biology, you will remember that we have 23 pairs of chromosomes. One of the pairs among the 23 is called an allosome or a sex chromosome which determines the sex of an individual, i.e., XX (female) and XY(male). While a female mammal receives 1 X chromosome from each parent, a male mammal receives 1 X chromosome from his mother and 1 Y chromosome from his father. Color vision anomalies arise on the X-chromosome. Since women have 2 X chromosomes, they carry the genes (also known as carriers) for color vision anomalies from their fathers and pass them on to their sons. Carrier women do not have any color vision problems as they have another intact X chromosome inherited from their mothers. This is also why more men have some form of color vision anomaly, as they only have 1 X chromosome. For a female to be color blind, the gene must appear on both her X chromosomes.
Similarly, a female becomes tetrachromatic when she receives a normal X chromosome from her mother and an X chromosome with a color vision anomaly from her father. She develops a 4th set of cone cells in her retina, making her a tetrachromat. Since a male receives only the Y chromosome from their father, they usually cannot be tetrachromatic.
Where else can we find tetrachromacy?
While tetrachromacy is rare among humans and mammals, it can be seen commonly among certain species of plants, fish, insects, reptiles, and birds. This feature helps find food and mates. Since plants and insects mutually benefit from being symbiotic, they have evolved complex colors to attract each other.
What extra colors do tetrachromats see?
As mentioned, tetrachromats can differentiate between 100 million colors. Some sources claim that tetrachromats can see ten colors in a rainbow. As the 4th cone sits between the m and l cones, tetrachromats can differentiate between more colors in the red, yellow, and green parts of the spectrum.
How can we test for tetrachromacy?
There is a lot of research ongoing to understand tetrachromacy and how to test it. Genetic testing is one of the best ways to profile your genes for this trait. The color matching test is also being used for confirming tetrachromacy in research labs. Researchers presented participants with a set of 2 mixtures of colors over a few sessions. Trichromats reported that they saw differences between the color mixtures. On the other hand, tetrachromats reported that the color mixture was the same in all the sessions.
An online test called the Derval color test claims to detect tetrachromacy:
If you see 20 or fewer colors in the above spectrum, you have dichromacy with only two functional cones. If you see between 20-32 colors, you are a trichromat with three functional cones, and if you can see between 33-39 colors, you are a tetrachromat. However, research shows that this test is inaccurate. According to researchers at Newcastle University in the UK, computers cannot generate the spectrum of colors detectable by tetrachromats. Dr. Gabriele Jordon, a neurobiologist researching tetrachromacy, says that while 12% of women may be tetrachromatic with four functional cones, only 2-3 % of them are actually functional. In fact, in her 20-years of research, she has come across only one tetrachromat who passed the match test with flying colors (pun intended). Known as subject cDa29, her story was a news flash in the media. Concetta Antico, an artist and tetrachromat, also shared her experiences about how tetrachromacy influences her art.
Research hopes to introduce new ways to test for tetrachromacy, especially by introducing new colors to observe if the fourth cone becomes functional in the other 10% of women. Maybe science can introduce a new set of colors beyond our imagination.