Genetics in Game of Thrones: Blood of the Dragon

I hope you had as much fun reading these as I did writing them. This will be the last one, unless the new book reveals some totally wacky inheritance or the new season seriously deviates from the books. And I’d like to talk about something that Daenerys Targaryen waxes poetic about for ages and ages: dragons.

I’m not going to talk about whether dragons are biologically plausible (short answer: they’re not). I’m going to talk about something which is in certain ways even less plausible: the idea that certain human families have heritable dragon-ness, which gives them an affinity to the beasts and a certain resistance to fire. This is explicitly stated of Daenerys: she can hang out in a funeral pyre overnight and be relatively unscathed. And she can, with a certain amount of effort and willpower, control Drogon. Viserys claims to have the trait but doesn’t, to horrifying repercussions. And certain other characters in later books either seem to have an affinity to dragons – although they haven’t been tested with fire as of yet – or think they will with, again, unfortunate repercussions.

There are two reasons I want to talk about this trait: first of all, it is by all appearances incredibly rare. It seems to be entirely linked to a single family, and only certain individuals within that family. Secondly, in a world with dragons, it would be incredibly advantageous. Not only the ‘less likely to be eaten by a dragon’ bit, which seems to me to be entirely relevant, but also the ‘fire-proof’ bit. So since it’s so advantageous, and since there are a certain number of people in Westeros claiming Targaryen blood, why aren’t more Westerosi fire-proof?

In the real world, why would an advantageous trait remain rare in a population?

One possibility is that it’s linked with a disadvantageous trait. In that case, the relative advantages and disadvantages balance each other out. The allele that causes sickle-cell anemia is an obvious example of this: in heterozygotes, it gives you resistance to malaria (a big advantage), and in homozygotes it gives you sickle cell anemia (a big disadvantage). And those two forces balance each other out: in African populations, where malaria is endemic, the selective advantage in heterozygotes was enough to make the allele common, but the disadvantage of sickle cell kept it from sweeping the population. In European populations, where malaria wasn’t as much an issue, the advantage in the heterozygote is removed and the allele is quite rare. In Daenerys’ case, maybe her infertility is linked to her affinity with dragons. In that, case, it would be a pretty strong selective force against the allele: since the advantage of not being eaten by dragons would be balanced out by the disadvantage of not being able to have kids.

But that gets really hairy really fast. First of all, it’s not just difficult for Dany to have kids: as far as she knows, it is flat-out impossible. And that trait, infertility, doesn’t seem to run in the Targaryen family (side note: infertility is one of those traits that rarely runs in families for somewhat obvious reasons). The only way this works at all for the phenotypes we’re sure of is if it’s almost exactly like sickle cell: you get fire-proofed with at least one copy of the dragon allele, and infertility if you have two. Which means both of Dany’s parents had to be carriers for the gene, and had to be fire-proof (possible, they’re both Targaryen by descent). And Viserys just got unlucky: zero copies, and you burn up.

There’s an obvious problem with this paradigm, and it’s what I talked about in my last post: the Targaryens are notoriously inbred. Which means they’re unlikely to have particularly many places where they’re heterozygous for anything. A strong selective force against homozygotes could lead to inbred populations retaining heterozygosity, but that would result in a lot of dead ends in the Targaryen family tree, which are notably absent.

But it turns out that the inbreeding which makes this balancing act unlikely can save the whole analogy: one other reason that an allele which is advantageous would stay within a small population is if that population is genetically isolated. Isolated by, say, only having children with one’s brothers and sisters.

And it turns out that, as far as I can tell from brief examination, a woman of house Targaryen marrying into another house happened exactly one documented time, when Daenerys Targaryen (the namesake, not the character we know) married into House Martell, to bring Dorne into the seven kingdoms under the Targaryens. (There’s some possibility of a Targaryen-relative marrying into house Baratheon at about the time of Aegon’s conquest as well.) There are some illegitimate children of various Aegons mentioned, but a family so apparently obsessed with purity of blood as the Targaryens could, believably, have had relatively few bastard children. And so the trait could well remain common among Targaryens and rare in the rest of the population, simply because Targaryens only marry Targaryens.

It’ll take another book, or two, before we know who, other than Daenerys and her ancestors, has enough Targaryen blood to ride a dragon. And it may turn out that ‘blood of the dragon’ is an emergent and not an inherited trait: that you don’t need a genetic link to old Valyria in order to have an affinity for fire-breathing lizards. (I’m holding out for Tyrion on a dragon, not because I think it’s particularly likely but because I have a special affinity for short, snarky people.) And maybe it’s all just magical. That was always a risk talking about science in Westeros. But with as many family trees as George R. R. Martin has created, it’s easy – and fun – to talk about genetics.

Previous posts: Paramutation, DNA Profiling, and Inbreeding