A world more colourful

By Marie Rogers

It is a beautiful day: the sky is blue, the trees are green and the poppies are red. I feel lucky to be a trichromat so I can appreciate these things.

Humans have three colour receptors in the retina which allow us to perceive the world in colour. These receptors are tuned to detect light wavelengths corresponding to blue, green and red. If one or more of these receptors is defective this leads to colour blindness.

Most mammals and primates only have 2 colour receptors, which allow discrimination between blue and yellow. It has been hypothesised that humans and a few other “old world” primates evolved a third receptor for red in order to forage for red berries in the green foliage.

However, some humans may be able to see even more colours. There is a small subset of women who have four distinct colour receptors, making them tetrachromats. The potential for tetrachromacy in women has long been recognised, because of the properties of the genes which code for colour receptors.

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Around 10% of men suffer from colour-blindness whilst this falls to less than 1% in women. This is because the genes which code for red and green colour receptors are located on the X chromosome. Women have two X chromosomes, therefore giving them ‘back-up’ coding genes for colour receptors if one goes wrong. Men have one X and one Y meaning that if the X is defective, there is no back-up.

Strangely, this genetic quirk may also give rise to females with extraordinary colour vision. During cell division to create sex cells, green and red colour receptor genes on each chromosome are mixed and a green gene may replace a red, or vice versa.

In males this would cause colour blindness but as females have a back-up this may result in 4 colour receptors being coded for. If the two green receptors are sensitive to a slightly different light wavelength (“shifted reception”) then the female may have extended colour vision.

Dr. Gabriele Jordan at Newcastle University tested the colour vision of women with colour blind sons, as they would be more likely to have 4 cones. One participant. “Mrs. M” could correctly distinguish between computer-generated tones which appeared identical to trichromats. This finding gives hope for the existence of functional human tetrachromacy and demonstrates how much simple genetic quirks can change an organism.

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