Re: Genetics of colour blindness?

On 7 Sep 2005 15:35:06 -0700, "rja.carnegie@xxxxxxxxxx"
<rja.carnegie@xxxxxxxxxx> wrote:

>This isn't particularly about creationism or refuting thereof, I just
>would like to know what intelligent and well-informed people can say
>about it.
>
>I got into a fan discussion on alt.fan.pratchett (Terry Pratchett,
>author of _Darwin's Watch_) about colour vision in e.g. bulls, dogs,
>and ourselves. I started reading some Web pages such as
>http://en.wikipedia.org/wiki/Color_blind and
>http://home.wanadoo.nl/paulschils/05.03.html and clearly it's awfully
>complicated.
>
>The latter page does assert strongly that bulls really are colour
>blind, and elsewhere I read that cattle don't have any cone cells in
>their eyes, which ought to settle that - but... It had (in effect)
>been put to me that bull colour-blindness was one of those things that
>"everyone knows" that isn't true. You know how hard it is to get those
>straight.
>
>Dogs, I'm not sure about now. "Everyone knows" about them, too, but
>another Wikipedia page (Dichromat) asserts that most mammal species
>typically have two sets of colour-sensitive cones, doesn't mention
>bulls, and discusses primates where there is a sex-related difference
>in three-colour vision or less, which I guess actually includes us...
>
>This is the part I don't have straight: according to Wikipedia, our
>genes for red and green cone cells are on the X chromosome. That means
>that a man is liable to be colour-blind if his one X has a broken gene,
>but a woman only gets it with two broken genes, which is way rarer.
>However, our blue colour gene is elsewhere, so when it does get broken,
>it only affects either sex if it's broken twice, once on each copy of
>the relevant chromosome.
>
>Now, colour vision seems kinda important and useful, so... if a
>location on the X chromosome is liable to cause colour-blindness in
>males, then wouldn't and shouldn't genetic copying error over the ages
>tend to put the red and green genes somewhere else safer, like the blue
>one? In other words, can we expect natural selection to weed out
>something like this if it's genuinely a disadvantage, or does it just
>go on the list of things that ought to be fixed sometime, no hurry,
>with the appendix and our feet arches and the rest?

Red-green color discrimination is a relatively recent evolutionary
development. Monkeys have it, but most mammals do not. (Dogs can
distinguish blues but not red/green, IIRC. Don't know about bulls.)
The green cone pigment gene, OPN1MW, is at locus Xq28, at the far end
of the X chromosome. This makes it particularly prone to damage.

For probably more than you ever wanted to know about this gene, see
http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=303800
OMIM at Entrez is a great resource; read at least enough to get a feel
the kind of stuff you can find there. For the lazy, I will quote a
particularly relevant paragraph:

"Nathans et al. (1986) isolated and sequenced genomic and cDNA clones
that encode each of the 3 visual pigments. The deduced amino acid
sequences show 41% identity with rhodopsin (180380). The red (303900)
and green pigments show 96% mutual identity, but only 43% identity
with the blue pigment (190900). Whereas there is a single red pigment
gene, green pigment genes vary in number among color-vision-normal
persons. The multiple green genes are arranged in a head-to-tail
tandem array. The evolutionary and other significance of this elegant
piece of work was outlined by Botstein (1986). The existence of
multiple green pigment genes in tandem array may explain why deutan
colorblindness is more frequent than protan colorblindness.
Furthermore, nonhomologous pairing and unequal crossing-over can
explain the development of colorblindness. Gene conversion may also be
involved. The green pigment genes vary in restriction pattern. In
determining which of the 3 opsin genes corresponded to each of the 3
cone pigments, Nathans et al. (1986) used the fact that 2 are X-linked
and 1 is autosomal. The autosomal one was clearly the blue cone
pigment which is defective in tritan colorblindness; which of the
other genes was for the red pigment and which for the green was
resolved by study of males with red or green colorblindness."

Botstein, D. : The molecular biology of color vision. (Editorial)
Science 232: 142-143, 1986.

Nathans, J.; Piantanida, T. P.; Eddy, R. L.; Shows, T. B.; Hogness, D.
S. : Molecular genetics of inherited variation in human color vision.
Science 232: 203-210, 1986.

--
Mark Isaak eciton (at) earthlink (dot) net
"Voice or no voice, the people can always be brought to the bidding of
the leaders. That is easy. All you have to do is tell them they are
being attacked, and denounce the pacifists for lack of patriotism and
exposing the country to danger." -- Hermann Goering

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