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Re: Monkey see (colors), Monkey do (was RE: Marsupials see colors)



"Thomas R. Holtz, Jr." <tholtz@geol.umd.edu> wrote:

> New World monkeys have odd sex-linked trichromatism (distinct from
> human sex-linked color-blindness: male color-blind humans are
> genetically trichromatic, apparently, but one of their opsins
> doesn't operate correctly).

I kind of wanted to gloss over the whole routine vs. polymorphic
trichromacy thing, but...  I just want to be clear here about the
difference between human sex-linked color deficiencies and the normal
situation in the majority of New World monkeys.  A normal human X
chromosome contains two distinct opsin genes.  Often one of them
exists as multiple copies.  The two genes are very similar to each
other.  Consequently unequal crossing over occurs with fair frequency
with the result that after meiosis, one or more of the genes is copied
incorrectly.  In some cases, the incorrect genes are still functional
opsins.  In other cases they are not.  The vast majority of females
have at least one of each of the "correct" genes.  A sizeable minority
(around 10%) of human males have at least one incorrect gene.  Since
they have only one X chromosome, males have only one chance to get it
right.  Their father's color vision status is irrelevant.  If their
mother has some form of color deficiency, so will the sons.  If she is
a carrier, they may inherit a deficiency.  If the deficiency results
from a functional hybrid gene, the person will still be a trichromat.
If the deficiency results from a non-functional gene, the person will
be a dichromat.

If by: 

> male color-blind humans are genetically trichromatic, apparently,
> but one of their opsins doesn't operate correctly).

Tom (or Dawkins?) is referring only to genetic structure and not to
functionality, then you could say that all mammals are at least
dichromats.  The marine mammals, owl monkeys, and procyonids I
mentioned in a previous post all have genes for two opsins.  It's just
that the S-cone opsin genes have premature stop codons which render
them non-functional.  Just like the opsin genes of humans with severe
forms of color deficiency.

> So a male [New World monkey] will have blue and either red or
> green. And females have blue and one of the following: only red,
> only green, or red and green.

I cringe at the use of color names in this context...  But it's
actually even more complicated than Tom describes.  The typical
scenario in New World monkeys is to have one S (derived from
"short-wave sensitive") gene locus with a gene fixed in the
population, and one M/L (from "medium-" and "long-wave sensitive")
gene locus with three possible alleles.  Let's call the M/L alleles A,
B and C.  There are thus nine possible genotypes.  All males are
hemizygous.  On their single X chromosome they have either A, or B, or
C.  Females can be AA, BB, CC, AB, BC, or AC.

> In howlers, a gene duplication and/or translocation has resulted
> with both R & G on X, making them trichromats.

In some sense they're better trichromats than we are.  The mechanisms
by which howler monkey opsin genes are expressed has not been fully
worked out, but unlike Old World primates with their single locus
control region, howlers duplicated the control and promoter regions as
well.

Denver Fowler <df9465@yahoo.co.uk> wrote:

} I thought most if not all birds possessed UV vision:

With all due respect, I'd say you thought wrong.  Look for review
articles by Hart, such as:

Hart, N. S. (2001).  "The Visual Ecology of Avian Photoreceptors",
     _Progress in Retinal and Eye Research_, 20(5):675-703.

Where you might be confused is that birds (like most non-primates)
generally have higher sensitivity than humans do to light with
wavelengths shorter than 400 nm.  That's because there are two reasons
for our insensitivity to short wavelengths, and in those birds not
considered to be primarily UV sensitive, only one of those reasons is
shared with us.  The reason shared by us and non-UV sensitive birds is
the spectral sensitivity of the photoreceptors.  Many birds (e.g.,
ducks and chickens) have their most UV-sensitive photoreceptors with
spectral sensitivities similar to the spectral sensitivity of human S
cones.  However, human corneas, lenses, and a layer of pigment
(macular pigment) over our central visual fields absorb a lot of short
wavelength light before it reaches the receptors.  Birds don't have
that sort of filter covering the relevant receptors, so even when
their receptors are similar to ours, they can see short wavelengths
better than us.  Or at least better than adults.  The UV-absorption by
human pre-retinal media is age dependent.  So chickens are UV
sensitive only in the same sense that human infants and toddlers are.

-- 
Mickey P. Rowe     (mrowe@lifesci.ucsb.edu)