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From: wen-king@cit-vlsi.Caltech.Edu (Wen-King Su)
Newsgroups: sci.med
Subject: Re: sex and color
Message-ID: <1137@cit-vax.Caltech.Edu>
Date: Sat, 8-Nov-86 02:16:17 EST
Article-I.D.: cit-vax.1137
Posted: Sat Nov  8 02:16:17 1986
Date-Received: Sun, 9-Nov-86 02:55:01 EST
References: <2302@bucse.bu-cs.BU.EDU> <7858@tekecs.TEK.COM>
Sender: news@cit-vax.Caltech.Edu
Reply-To: wen-king@cit-vlsi.UUCP (Wen-King Su)
Organization: California Institute of Technology
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In article <7858@tekecs.TEK.COM> mikes@tekecs.TEK.COM (Michael Sellers) writes:
>In article <2302@bucse.bu-cs.BU.EDU>, gasp@bu-cs.BU.EDU (Isaac Kohane) writes:
>> Well, it seems that women can in fact see more than men.
>> [...]
>> Now, as we all know, women have twice as much X chromosome as men do.
>> Therefore, if there are multiple alleles (versions of that gene at
>> that locus) for the red pigment (as is suggested by recent experiments
>> in this area), then a women could have one allele on one X chromosome,
>> and another allele on the second X chromosome. Each allele would have
>> a different peak absorption wavelength. It might be then, that some
>> women are in fact quadrichromatic. What do they see that we don't? :-)
>
>If I remember correctly, the second X chromosome in females sits unused in
>the nucleus (it never "unbundles" in interphase like the rest of the 
>chromosomes do).  These are called "Barr bodies", after the guy who 
>discovered them.  If my recollection is correct, then this would seem to 
>imply that females do not benefit from anything on the "extra" X chromosome.
>Can anyone closer to a genetics text confirm or deny my remembrance?
>

The red/green color blindness in human is caused by a defect in this gene.
The low incidence of red/green color blindness among women is attributed
to the fact that women have two alleles and that the chance of having two
defective alleles at the same time is quite small.  This seems to conclude
that both of the X chromosomes can be expressed.  However, I believe only
one of them can be expressed in a cell.  Certain genes for skin color and
hair color can be founded on the X chromosome.  If the two alleles of a 
skin color gene are sufficiently different and if different X chromosomes are
expressed at different parts of the body, the individual would have patches
of different colors over the body.  I do not remember seeing this condition
in human, but it is known in cats and is found only in female cats.  Does
anybody know of a human example?  Is there anybody out there with a normal
left eye and a color blinded right eye?

As long as both of the X chromosomes can be expressed, whether they can
both be expressed in the same cell or not, I believe if a women has two
different alleles for the gene, she is going to have both type of the
red pigment in her eyes.