Colourblind island: genetics
Jun. 24th, 2011 11:30 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
steorran_woruldeThere'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.
Let's call the gene that causes rod monochromacy r. -r is the non-colourblind variant, while +r is the colourblind variant. A combination of these genes which causes rod monochromacy counts as +R while a combination that doesn't counts as -R. People can have the following combinations:
-r -r: two dominant colour-seeing variants. Unaffected. -R
-r +r: one dominant colour-seeing variant, one recessive colourblind variant. Carrier. -R
+r +r: two recessive colourblind variants. Affected. +R
Let's call the gene that causes blue cone monochromacy bX. -bX is the non-colourblind variant, while +bX is the colourblind variant. A combination of these genes which causes blue cone monochromacy counts as +B while a combination that doesn't counts as -B. People can have the following combinations:
-bX -bX: two dominant X-linked colour-seeing variants. Unaffected female. -B
-bX +bX: one dominant X-linked colour-seeing variant, one recessive X-linked colourblind variant. Carrier female. -B
+bX +bX: two recessive X-linked colourblind variants. Affected female. +B
-bX Y: one X-linked colour-seeing variant. Unaffected male. -B
+bX Y: one X-linked colourblind variant. Affected male. +B
The next question is, how do these two traits interact?
I think that +B prevents/inactivates red and green cones (while leaving blue cones intact), while +R prevents/inactivates all cones, regardless of colour. And the genetics of R and B are independent, since they're in different locations. So I'd expect combinations of R and B to work as follows:
-R -B: Colour-seeing; no inactivation of cones.
-R +B: Blue cone monochromat: red and green cones are inactivated by +B but -R doesn't inactivate all cones, so blue cones still function.
+R +B: Rod monochromat: +R inactivates all cones, so the fact that +B would additionally inactivate red and green cones is irrelevant.
+R -B: Rod monochromat: +R inactivates all cones, so the fact that -B would leave all cones unaffected is irrelevant.
So what does this mean for my island?
Well, if everyone on the island was +R (so that only recessive +r and not dominant -r was in the genetic mix), there would be no blue cone monochromats on the island.
If there was a +R population (with only the +r variant), it could have both +bX and -bX variants active, and everyone would be rod monochromats; there would be no blue cone monochromats.
If there was a population where everyone was blue cone monochromats (and there were no rod monochromats), everyone would have only the +bX variant, but everyone would have at least one copy of -r (and, if this were to persist over multiple generations, with no rod monochromacy in the mix, everyone would have two copies of -r). More realistic is a population of primarily blue cone monochromats, where -r is the commonest variant but +r is also in the genetic mix, creating some rod monochromats and some carriers of rod monochromacy. In any case, -r is strong in the mix, because it has to be present for someone to be a blue cone monochromat rather than a rod monochromat.
When these two populations come in contact, you could end up with people who have -bX (from descent from the +R population which has both -bX and +bX in the mix) and who also have -r (from descent from the blue monochromat population). These people would be colour-seers.
So for my island to work as a colourblind island, it can't have two such populations in contact.
One solution is to scrap the idea of some families with cone monochromacy, and just have the whole island be rod monochromats. That's fairly easy.
The other solution, except I haven't yet come up with a plausible historical scenario for it to originate from, is:
For bX, the only variant in the population is +bX. There is no -bX in the population. Thus the whole population is +B, and everyone will be colourblind. For r, the vastly most common variant is +r, so that the vast majority of people are rod monochromats (+R). However, there are a few families with -r, which allows active cones to form in people with at least one copy; since +B is still present and preventing red and green cones from being active, these people will be blue cone monochromats.
I might just go with that solution even if I don't have the historical source figured out, because it achieves the population arrangement that I wanted.
[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.
Let's call the gene that causes rod monochromacy r. -r is the non-colourblind variant, while +r is the colourblind variant. A combination of these genes which causes rod monochromacy counts as +R while a combination that doesn't counts as -R. People can have the following combinations:
-r -r: two dominant colour-seeing variants. Unaffected. -R
-r +r: one dominant colour-seeing variant, one recessive colourblind variant. Carrier. -R
+r +r: two recessive colourblind variants. Affected. +R
Let's call the gene that causes blue cone monochromacy bX. -bX is the non-colourblind variant, while +bX is the colourblind variant. A combination of these genes which causes blue cone monochromacy counts as +B while a combination that doesn't counts as -B. People can have the following combinations:
-bX -bX: two dominant X-linked colour-seeing variants. Unaffected female. -B
-bX +bX: one dominant X-linked colour-seeing variant, one recessive X-linked colourblind variant. Carrier female. -B
+bX +bX: two recessive X-linked colourblind variants. Affected female. +B
-bX Y: one X-linked colour-seeing variant. Unaffected male. -B
+bX Y: one X-linked colourblind variant. Affected male. +B
The next question is, how do these two traits interact?
I think that +B prevents/inactivates red and green cones (while leaving blue cones intact), while +R prevents/inactivates all cones, regardless of colour. And the genetics of R and B are independent, since they're in different locations. So I'd expect combinations of R and B to work as follows:
-R -B: Colour-seeing; no inactivation of cones.
-R +B: Blue cone monochromat: red and green cones are inactivated by +B but -R doesn't inactivate all cones, so blue cones still function.
+R +B: Rod monochromat: +R inactivates all cones, so the fact that +B would additionally inactivate red and green cones is irrelevant.
+R -B: Rod monochromat: +R inactivates all cones, so the fact that -B would leave all cones unaffected is irrelevant.
So what does this mean for my island?
Well, if everyone on the island was +R (so that only recessive +r and not dominant -r was in the genetic mix), there would be no blue cone monochromats on the island.
If there was a +R population (with only the +r variant), it could have both +bX and -bX variants active, and everyone would be rod monochromats; there would be no blue cone monochromats.
If there was a population where everyone was blue cone monochromats (and there were no rod monochromats), everyone would have only the +bX variant, but everyone would have at least one copy of -r (and, if this were to persist over multiple generations, with no rod monochromacy in the mix, everyone would have two copies of -r). More realistic is a population of primarily blue cone monochromats, where -r is the commonest variant but +r is also in the genetic mix, creating some rod monochromats and some carriers of rod monochromacy. In any case, -r is strong in the mix, because it has to be present for someone to be a blue cone monochromat rather than a rod monochromat.
When these two populations come in contact, you could end up with people who have -bX (from descent from the +R population which has both -bX and +bX in the mix) and who also have -r (from descent from the blue monochromat population). These people would be colour-seers.
So for my island to work as a colourblind island, it can't have two such populations in contact.
One solution is to scrap the idea of some families with cone monochromacy, and just have the whole island be rod monochromats. That's fairly easy.
The other solution, except I haven't yet come up with a plausible historical scenario for it to originate from, is:
For bX, the only variant in the population is +bX. There is no -bX in the population. Thus the whole population is +B, and everyone will be colourblind. For r, the vastly most common variant is +r, so that the vast majority of people are rod monochromats (+R). However, there are a few families with -r, which allows active cones to form in people with at least one copy; since +B is still present and preventing red and green cones from being active, these people will be blue cone monochromats.
I might just go with that solution even if I don't have the historical source figured out, because it achieves the population arrangement that I wanted.
[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.