Re: Genetics of colour blindness?


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?
>
> If natural selection /should/ have improved on a disadvantageous
> arrangement, then I suppose we look for reasons it may /not/ be
> disadvantageous, so that selection leaves well alone.
>
> So far I've hung out a facile argument going back to our
> hunter-gatherer days. Suppose men are hunters (dealing with prey
> species as well as predators) and women are gatherers (fruit and so
> forth). I suppose hunter-gatherer doesn't necessarily even mean that,
> but never mind. The argument is that the fruit-pickers need good
> colour vision, but the hunters don't.
>
> What's more, apparently there is, in fact, an interesting effect when
> mutations in the cone colour genes come together in a pair of X
> chromosomes: you can have /more/ than three different colour responses,
> and apparently this does give some people, mostly women, a more subtle
> sense of colour than the usual. I'm not too clear on how the brain is
> adapted to recognise that there are four or more colours out there
> instead of the expected three, but then I guess that assumes three is
> normal. Maybe not?
>
> Conversely, Wikipedia gives name and date to a story that colour-blind
> guys can see through camouflage that stops a normally sighted guy. At
> a guess, the reverse is more often true. But suppose a hunting party
> goes out with one or two colour blind guys, then you get the best of
> both worlds.
>
> (The second URL I listed has some colour tests where you only see a
> pattern if you're colour-blind. Incidentally, I think those tests
> depend on the quality of colour on your computer monitor [nat'rally],
> and I wonder... I found some of the "hard to read if you're
> colour-blind" tests hard to read, but I did get the right answers.)
>
> So, bottom line: is red-green colour-blindness sex-linked for a reason?
>
> I know - we have reasons, we have theories, biology does without both
> and just gets on whatever it feels like doing...

The thing about biological evolution is that it is historical and
builds on what came before. Species evolve from preexisting species.
They have to work with the junk that their ancestors stuck them with.

Chromosomal sex determination is one of the means that some species
have developed as a means for determining sex. There seems to be
multiple independent developments of this method in different lineages.
A lot of mammals have the XY system, but not all. Birds have the ZW
where the females are the heterogametic sex (ZW) and the males are ZZ.
The Z and X chromosome come from different origins and have different
genes on them. The mammal that developed the XY system happened to
have some of the genes for color vision on the sex chromosome. Birds
did not. What are you going to make out of something that is probably
just an accident of history?

Ron Okimoto

.

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