Rare Disease Report

A Good Mutation: Seeing the World With Extra Cones

DECEMBER 25, 2016
Ricki Lewis, PhD
Maureen Seaberg knew she had a rare gift long before genetic testing confirmed it in 2013. She’s a tetrachromat: an extra (fourth) type of cone gene enables her to see things the rest of us can’t. And although up to 12% of all women may share her genetic gift, thanks to environmental influences, very few know it.

The Girl With Kaleidoscope Eyes

“Very frequently I’ll have polite disagreements with people about what color things are. It’s like living the “what color is the dress” argument your entire life.” She’s had issues with a friend calling a gourmet carrot red when it’s clearly beet-colored, balked at the imprecision of the computerized paint mixer at Home Depot, and grown frustrated at works of art that aren’t as good as they could be. “Time and again I’ll look at a print or textile and think it’s not what the artist meant!” A mere trichromat, who has three types of cones, can suspect when color is off in an item of clothing, Maureen adds, but she can actually see the defect.
Once in awhile Maureen’s peculiar sense of color manifests as a wardrobe warning: a dress that is a beautiful salmon color to everyone else appears to her more like a rotting fish, due to hidden murky undertones. But most of her exceptional visual experiences come from nature. “For example, the shade when sunset is just beginning is both pink and a sparkly champagne, without every becoming peach. A piece of glass is pale green and brown, but not khaki. Another color I see is when the sun shines brightly on trees, and some of them appear a silvery white green,” she says. Maureen thinks the visual world extends beyond our planet. “The Hubble photos from space hint that this is a far greater palette than a terrestrial can see, even a functional tetrachromat like me. Computer monitors and TVs are not equipped to translate that,” she explains.
Back on Earth, so distinctive are the hues of Maureen’s world, and so vivid her descriptions of what she sees, that a major cosmetic company tapped her enhanced eyes for a new line of lipsticks.
Maureen also has synesthesia, and that’s how we began chatting (in her Psychology Today column) a few years ago, because I have it, too. In synesthesia, senses overlap. For me, days of the week have specific colors and textures. Maureen sees colored days, weeks, and months, but her numbers and letters are also colored, and she has mirror touch empathy—she can feel what others feel.
Maureen is the author of several terrific books, including Struck by Genius, How a Brain Injury Made Me a Mathematical Marvel and Tasting the Universe: People Who See Colors in Words and Rainbows in Symphonies.

An Extra, Mosaic Opsin Gene

Tetrachromats typically have male relatives who are colorblind because they are missing a cone type. When Maureen contacted Jay Neitz, Bishop Professor of Vision Sciences at the University of Washington and the researcher who has genetically corrected colorblindness in monkeys, and mentioned her three colorblind male relatives, he sent her a spit kit to probe her cone genes.
Humans and other apes are “trichromats”, three cone genes encoding proteins, called opsins, that capture light of specific wavelengths. A “blue” opsin gene on human chromosome 7 captures short wavelengths of light, while the “green” gene captures middle wavelengths and the “red” gene long wavelengths, both on the X chromosome. Because the two X-linked opsin genes are similar in DNA base sequence, replicating DNA can misalign, and if the two X’s cross over, create a third opsin gene that’s a mosaic of parts (exons) of the red and green genes.

Such an over-endowed X chromosome in a male, who has only one X, doesn’t seem to do very much. But if a female inherits an X with three cones genes, she can be a functional tetrachromat like Maureen, seeing 100 million shades compared to the normal mere million.

More Than Genetics

Psychologist Kimberly Jameson, who runs the Human Tetrachromacy Research Collaborative at UC Irvine, thinks that two traits besides an extra cone gene are necessary for tetrachromacy: lifelong exposure to color, such as being an artist or a synesthete; and enhanced vocabulary (Maureen says she was “obnoxiously verbal” as a child). It’s also about opportunity. “If a woman has a genetic potential for tetrachromacy and is a painter, but is educationally impoverished, will she have the strong vocabulary necessary? Is it my vocabulary that allows me to see beet color when someone else sees just red?” says Maureen.

Another factor in experiencing tetrachromacy is X inactivation, which turns off one X chromosome in every cell of a female mammal, points out Gabriele Jordan, who heads the Tetrachromacy Project at Newcastle University. Tetrachromacy might require a certain minimum percentage of X chromosomes bearing the unusual compound gene to remain activated in cone cells. The pattern of the special cones might also be a factor.

Dr. Jordan and her colleagues tested 25 tetrachromats identified from family history of colorblindness and genetic testing. The women took a Rayleigh test, which blends pairs of lights to match third colors. The test revealed functional tetrachromacy for only one of the 25 women, who demonstrated “flawless discrimination,” the researchers write in the Journal of Vision.

An Evolutionary Enhancement?

Most primates are dichromats—they have only two cone types. Jay Neitz has given squirrel monkeys extra opsin genes to model how a mutation in an old-world female monkey about 30 to 60 million years ago may have led to the tri-color vision that preceded the explosion of primate diversification.
Maureen imagines how color vision might have sped human evolution. “Our primate ancestors could spot predators better, and discern fresh and spoiled fruit with greater precision. For example, a tetrachromat might be able to spot the initial flush of fever on a child’s face.“ She recalls years ago visiting a relative whom she hadn’t seen in awhile and noticing a slight shininess and pink discoloration in skin patches on his face. They turned out to be precancerous lesions!
Tetrachromacy proved adaptive for Maureen in another situation – weather prediction. “I was standing outside with a group of people and saw the sky become slightly green. And so I urged them to go indoors, because something bad was coming. Then tornado winds picked up.”
Perhaps evolution will make us all tetrachromats, given millions of years, a lot of sex, and a clear advantage to seeing those sparkling sunsets and hues of rotting food. Although Maureen would like to be “a bit of a change agent, as in synesthesia, to get more women to come forward and get testing,” she is inadvertently hindering human evolution with her lipstick line. Although I don’t think that cosmetics influenced Charles Darwin’s idea of sexual selection, Maureen’s providing color guidance to others may counter natural selection of her genetic gift.
It’s nice to know that mutations aren’t all bad.

Stay informed on the latest rare disease news and developments by signing up for our newsletter.
Copyright © RareDR 2013-2019 Rare Disease Communications. All Rights Reserved.