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Cambridge University Science Magazine
The human eye can normally resolve millions of colours, a practically

continuous spectrum of wavelengths, with sensors for only red, green

and blue. Each of these photopigments is sensitive to a wide range of

colours, but their sensitivities are peaked at the respective

wavelengths. As a result, every colour (wavelength) corresponds to a

unique combination of relative signal strengths from the three

sensors. However, in colour blindness, usually one of the

photopigments is missing, and the two remaining signals no longer

determine a single colour.

The genes for the red and green photopigments are located in the X

chromosome, which is why men are much more likely to be colour blind

than women. The interesting mutation of tetrachromacy arises from the

fact that the red, green and blue wavelengths vary slightly between

individuals. Then, the two X chromosomes from both parents could have

genes for slightly different pairs of red and green photopigments. A

phenomenon called X inactivation may cause some cells to derive from

one X chromosome and other cells from the other. The result may be a

tetrachromat, a person with a fourth photopigment between red and


It might be expected that this automatically leads to increased

resolution of colours. However, it has been a matter of much

discussion and experiment whether the brain can make use of the extra

colour signals. Also, the above described mechanism behind

tetrachromacy implies that the fourth photopigment is often very close

to either red or green, and the effect would be hardly noticeable.

Nevertheless, ''Mrs. M'' who took part in experiments led by Gabriele

Jordan in 1993, is a living proof of the theory, with exceptionally

accurate colour vision. "People will think things match, but I can see they don''t."

Read the full article here.

Risto A. Paju is a Undergraduate in Physics at Queens'