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From: vangeldr@cmgm.Stanford.EDU (Russ Van Gelder)
Subject: Re: Primary colors in human color vision
Message-ID: <1991Mar25.233229.15316@medisg.Stanford.EDU>
Summary: Polymorphism leading to a four-color system... 
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References: <00945FE5.1F9B5480@aclcb.purdue.edu> <1991Mar23.193006.22992@pinhead.pegasus.com> <1991Mar24.002117.24100@medisg.Stanford.EDU> <5365@archive.BBN.COM>
Date: Mon, 25 Mar 91 23:32:29 GMT
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The suggestion that allelic polymorphism could lead to a
"tetrachromacy" is interesting; however, it would take a novel
mechanism to produce such a beast.  The trichromacy system works
because 1.) each cone only produces one of the three color pigments,
and 2.) its connections are segregated by color; in fact, in the
highest density region of the fovea, each cone synapses on only a
single bipolar cell. 

First, men are not affected by such polymorphisms for the red and
green loci, since they are X-linked.  Although a given man may have up
to four copies of the green (I think) gene in a tandem array, and
although these may show variation, I don't think that this variation
would be propagated to individual cones, and all cones expressing the
same suite of genes would have the same spectral response.  I don't
think this would lead to the ability to detect new colors.

In women, the story is more interesting.  A woman can be heterozygous
for the color pigments (which is why they rarely have red-green color
blindness); but each individual cell inactivates one of the X
chromosomes, making the retina a chimera of each of the genes.  Thus,
a woman could have functional segregation of four or more
photoreceptors.  However, in order for this information to be
processed, the higher order neurons in the pathway would have to have
a way to know *which* X was inactivated; perhaps this could be through
some sort of an activity-dependent process.  

As for the blue pigment, which is autosomal, a mechanism to increase
the number of photoreceptors would have to include allelic exclusion,
such that only one allele of a heterozygote would be expressed.
Although such a mechanism exists in the immune system, to my knowledge
it is not utilized in any other autosomes.

Russ