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Colourblind island: genetics
Jun. 24th, 2011 11:30 amThere's an island on Domil - a fairly large island, perhaps comparable in size to Ireland, perhaps not that big - where the whole population is totally colourblind. Most of the population is made up of rod monochomats (people who have only rods, no cones, in their eyes). I've long wanted to arrange it so that some families on the less-accessible northern coast of the island are (or contain) blue cone monochromats (people who have rods and blue cones in their eyes, but can't perceive colour since they don't have contrasting varieties of cones. [1])
However, I've never been sure whether it would be genetically plausible to have both those varieties of colourblindness in a population without also having some people with colour vision. The other day I found out a little more about the genetics of colourblindness. (I don't know very much about genetics, so I may use terminology wrongly or make other mistakes. Please correct me if so, especially if I'm making mistakes that will change the effects of genetics on my world.)
According to this page, rod monochromacy is autosomal recessive, encoded either on chromosome 2 or chromosome 8. I'll make the simplifying assumption that only one location is active in the population I'm working with. Recessive is good, because that means that in a population where everyone has the trait, then everyone has only the gene variant that causes the trait. (If the colourblindness gene variant were dominant, then there'd be a recessive colour vision gene variant that could be floating around in a population even when everyone had the coloublindness trait, and then colour vision could pop up in a subsequent generation.)
According to the same page, blue cone monochromacy (lacking both red and green cones) is an X-linked (recessive) trait just as the two basic kinds of red-green colourblindness are (red cone lack/diminished function and green cone lack/diminished function). Another page from the same site discusses in more detail how this works for red-green colourblindness, and so by extension for blue cone monochromacy. This also works out so that if the whole population has the trait, then everyone has only the gene variant that causes the trait.
( This gets long )
[1] Actually, blue cone monochromats may have a very small amount of colour perception based on the difference in wavelengths to which rods and blue cones are sensitive, especially around twilight. But it's very small, and I'm guessing it's minor enough that a vocabulary for these colours would likely not be developed.
However, I've never been sure whether it would be genetically plausible to have both those varieties of colourblindness in a population without also having some people with colour vision. The other day I found out a little more about the genetics of colourblindness. (I don't know very much about genetics, so I may use terminology wrongly or make other mistakes. Please correct me if so, especially if I'm making mistakes that will change the effects of genetics on my world.)
According to this page, rod monochromacy is autosomal recessive, encoded either on chromosome 2 or chromosome 8. I'll make the simplifying assumption that only one location is active in the population I'm working with. Recessive is good, because that means that in a population where everyone has the trait, then everyone has only the gene variant that causes the trait. (If the colourblindness gene variant were dominant, then there'd be a recessive colour vision gene variant that could be floating around in a population even when everyone had the coloublindness trait, and then colour vision could pop up in a subsequent generation.)
According to the same page, blue cone monochromacy (lacking both red and green cones) is an X-linked (recessive) trait just as the two basic kinds of red-green colourblindness are (red cone lack/diminished function and green cone lack/diminished function). Another page from the same site discusses in more detail how this works for red-green colourblindness, and so by extension for blue cone monochromacy. This also works out so that if the whole population has the trait, then everyone has only the gene variant that causes the trait.
( This gets long )
[1] Actually, blue cone monochromats may have a very small amount of colour perception based on the difference in wavelengths to which rods and blue cones are sensitive, especially around twilight. But it's very small, and I'm guessing it's minor enough that a vocabulary for these colours would likely not be developed.