You Are What You Choose To Do Repeatedly

That said, there are exceptions. Mendelian traits (Characteristics that are influenced by alles at only one genetic locus) do exist in humans, a number of them being related to genetic diseases. The list in the ANTH 102 notes I just dug up was: Wet (dominant) or dry Earwax; Albinism; Brachydactyly (dominant); Blood type (ABO, not the positive/negative part); Hereditary breast-ovarian cancer syndrome (BRCA-1, BRCA-2, unknown genes); Huntington’s disease; Lactase persistence (dominant); and Sickle-cell disease (recessive).

So the sickle cell punnet square looks like this for two parents who both have one copy of the sickle cell gene:

Sickle cell is a very painful and life threatening disease, (That's why the FDA approving a crispr treatment for it allowing patients to be their own bone marrow donors is very exciting.) and from an evolutionary perspective, one that very often prevents people from reproducing. It's also not strictly dominant/recessive, in that people heterozygous for sickle cell can have some symptoms like the possibility a sickle-cell crisis triggered in low-oxygen situations (high altitudes, intense exercise, etc).

So one might think that Sickle cell would be a vanishingly rare disease, since having it can be deadly and even having the trait can in some cases cause problems. Only it's not rare by genetically inherited disease standards, not at all.

And to make a long story very short, the reason is malaria.

Malaria

People who are heterozygous (possessing one sickle cell gene and one normal gene) for sickle cell anemia are resistant to malaria. In areas of the world without a high incidence of malaria historically, there is a strong selection against the sickle cell gene, (Biointeractive Malaria and Sickle Cell Anemia, 9:33) but in areas with malaria, both having sickle cell disease (homozygous HbSS) and not having the trait at all (homozygous HbAA) are selected against. People with sickle cell were historically less likely to reproduce, and people who were not resistant to malaria were more likely to die of malaria and also not reproduce. Because being heterozygous with sickle cell is selected for, the gene persists in the population.

The implications of that are best summed up from this map that I just stole from Britannica.com:

I dunno if the percentages on that second one are accurate tbh, other infographic maps I'm looking at give different ranges. but sill, you get the gist about how common it is in equatorial Africa. In the modern United States Black children are much much more likely to be born with sickle cell than white children—the genes don't just go away when the threat of malaria is removed. (And yeah, that's a historical consequence of the slave trade.)

There's some other stuff wrapped up in here too about bio-cultural evolution: There's indications that malaria was not a “significant problem until humans abandoned food foraging for farming” (Haviland, W. A., Prins, H. E., Walrath, D., & McBride, B.). Humans cleared away the forest, which had kept the soil absorbent. Without the vegetation, more water built up on the surface, forming stagnant puddles which were a perfect environment for malaria-causing mosquitoes to thrive, thus creating the conditions for sickle cell anemia to be advantageous. Farming creates caloric surplus which is great for humans, but it also changes the environment in ways that can be detrimental. Malaria is one way, creating the conditions for other epidemic diseases to thrive is another, etc. etc.

But if you've read this far you're probably going "Chi, you promised this would be a fandom post but so far this has been a serious and kind of sad post about disease. when are you going to get to the dragons?"

The Dragons

So the first time malaria comes up in the Temeraire books is in Throne of Jade, when a bunch of the sailors on the Allegiance come down with "malarial Fevers."

Jane, I must ask you to forgive the long gap in this Letter, and the few hasty Words that are all by which I can amend the same now. I have not had Leisure to take up my pen these three weeks—since we passed out of Banka Strait we have been much afflicted by malarial Fevers. I have escaped sickness myself, and most of my men, for which Keynes opines we must be grateful to Temeraire, believing that the heat of his body in some wise dispels the Miasmas which cause the ague, and our close association thus affords some protection. But we have been spared only to increase of Labor: Captain Riley has been confined to his bed since almost the very first, and Lord Purbeck falling ill, I have stood watch in turn with the ship’s third and fourth lieutenants, Franks and Beckett. Both are willing young men, and Franks does his best, but is by no means yet prepared for the Duty of overseeing so vast a Ship as the Allegiance, nor to maintain discipline among her Crew—stammers, I am sorry to say, which explains his seeming Rudeness at table, which I had earlier remarked upon.

I do not know enough about what people thought about malaria in the 19th century to be 100% sure that this is actually malaria, but I think Novik wouldn't want to confuse her readers by calling something malarial that isn't you know..malaria. So I'm going to assume thats what it is. Google is not giving me figures on malaria survival rates before modern medicines for it which is driving me kind of nuts and means I can't say how lucky Riley and Purbeck were to survive with apparently no complications, but that's not the point here anyway. The point is the comment about the aviators not getting sick.

And not only (mostly) not getting sick, but not getting sick even though they aren't actually always near Temeraire. Laurence for example has been working watch shifts near constantly because he's the only one left on the ship who knows what he's doing. That means probably less read & cuddle time than is normal for him and Temeraire, and yet—no malaria.

We modern readers (and Novik) know that malaria is not caused by "miasmas" but by parasites carried by mosquitos. And lo and behold when we get to Empire of Ivory we get:

Mosquitoes sang happily as dusk drew on, though they did not come very close to Temeraire; the flies were less judicious. The shapes of the trees were growing vague when Temeraire woke with a start and said, “Laurence, there is someone coming, there,” and the grass rustled on the opposite bank.

So yeah, the dragons are keeping the mosquitos away. I know fuck all about why—it's probably not heat since you know, mosquitos like warm blooded organisms, but maybe it's an oil or a chemical or some artifact of the way some of them can breathe fire that's present in all dragons or something, they're described as smelling weird a few times, so who knows. If it's a substance like an oil in their skin that could explain why the aviators don't get sick even when they're not nearby, since they could have some on them from contact, but that's just speculation. The point is not the mechanism, just that it's happening.

The Point

This whole post grew out of a throwaway comment I made about the benefits of mutualistic symbiosis with dragons from the human perspective in that one post about how the series has some interesting stuff obviously going on psychologically/biologically. The point of going in-depth on malaria and sickle cell is to show how this is really impressively solid worldbuilding in relation to the Tswana.

See, Empire of Ivory describes locations that seem like they're in modern day Botswana, Zimbabwe, and Zambia, regions which will have had long-term problems with malaria-causing mosquitos. That's not the densest area for sickle cell, but still definitely in the region where malaria would have exerted selective pressure.

Selective pressure which, in a universe where just being around a dragon is going to drastically reduce malaria rates, is going to leave dragon-friendly populations a lot healthier than dragon-unfriendly ones. A community that has a dragon stay every night and work alongside humans during the day is going to have a lot less malaria even without the sickle cell resistance than a community which has no dragon. And considering that malaria is bad enough that sickle cell genes persist despite it also having a high chance to cause a deadly disease, whereas a dragon that's a fully prosocial member of the community is not going to cause more death and instead will probably help with defense and create more caloric surplus (at the cost of consuming most of that surplus) a dragon is just obviously the better option. From there, it's extremely easy to see how the Tswana in the series could develop such a dragon-centric culture and have it be so wildly successful. The dragons provide fertilizer, the dragons allow for fully domesticated elephants, and the dragons render malaria—one of the deadliest diseases in history—nearly a nonissue. Of course they're family.

Citations:

Biointeractive. (2014, August 26). Malaria and Sickle Cell Anemia - HHMI BioInteractive Video. Retrieved October 3, 2018, from https://www.youtube.com/watch?v=Zsbhvl2nVNE