TUESDAY, 5 DECEMBER 2000
The human eye can normally resolve millions of colours, a practicallycontinuous 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
green.
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'
