Fun_People Archive
11 Jan
Colour Art & Science

Date: Thu, 11 Jan 96 16:13:51 -0800
From: Peter Langston <psl>
To: Fun_People
Subject: Colour Art & Science

Forwarded-by: bostic@bsdi.com (Keith Bostic)
Forwarded-by: "Rob Pike" <rob@plan9.att.com>

i've been reading this fascinating book called 'colour art & science', a
collection of articles about color published by cambridge university
press. it's just packed with amazing facts.  here are a few tidbits,
discovered very recently.

tricolor vision - what people (erroneously, i learned) think of as red,
green, and blue light receptors in the eye - is not a common mammalian
trait.  it evolved fairly recently in old world monkeys.  with the old
two-color system, there was little distinction between red and green, only
between `long' and `short' wavelengths.  birds have good color vision,
which means that parrots, for example, can see red fruit in the jungle
better than most mammals.  about 30 million years ago, fruits from some
flowering trees in the jungle started getting bigger - up to about 50
grams, too large for birds - and redder.  simultaneously, by changing just
a few amino acids in the long-wave color receptor, monkeys evolved a new
visual detector that could be used to discriminate red from green.  thus,
our visual system evolved, quite specifically, to see fruit in the jungle.

the trees are the true beneficiaries, of course, since we eat the fruit
and spread the seeds.  thus one could even say that our color vision
evolved to help fruit trees propagate.

things like red-green color blindness are caused by hiccups in this
system.  the genes are carried only on the X chromosome, so men are more
susceptible since they get only one copy of the gene.  several things can
happen; the most common is cross-linking during duplication of the gene
causing the new pigment to be shifted only slightly in wavelength from
the old pigment; since they are so similar anyway, cross-linking causes
fewer of the small number of changes to be passed on correctly to the
child.  thus the wavelength of the redder pigment is not sufficiently
shifted from the green.  (the story is more complicated than this, but
you get the idea.)  about 8% of men suffer some form of this; almost no
women.

here's my favorite.  ever wonder if other people see the same colors you
do?  color blindness aside, they may not.  there are several different
encodings of the long-wave pigment.  in particular, 60% of white males
code for serine at position 180; 40% of men code for alanine.  serine
produces a longer-wavelength pigment.  the difference can be measured by
asking men to match colors by mixing red and green; the two populations
will choose different mixtures to match, say, orange.  the match chosen
by one population will look distinctly off to the other.

as the author says in the book, this is the first proven example of 'a
variation in our mental worlds [being] traceable to a polymorphism - a
single variation - in our genes'.

© 1996 Peter Langston