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​The Weird Rules of Genetic Inheritance in 'Game of Thrones'

The seed is inconsistently strong.
Image: HBO

SPOILER ALERT: This article contains spoilers for all five seasons of Game of Thrones, but does not include A Song of Ice & Fire spoilers.

The season five finale of Game of Thrones premiered last night, and boy, was it a rough one, even by the show's brutal standards. From penance walks to poisoned princesses to "for the watch," it seems that the world of ice and fire has never been so bleak.

In the end, Jon Snow was downed by the same commitment to honor that robbed Ned Stark of his head all the way back in season one. But while Jon was axed for making common cause with the wildlings, Ned was executed for discovering the true meaning of his mentor Jon Arryn's last words: "The seed is strong."

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Arryn probably should have been less cryptic in his final hours, and just said, "the Baratheon gene for black hair is super duper dominant." This is, after all, the revelation that throws the paternity of Robert's three platinum blonde children into question, and reveals that the genetic mechanisms determining inherited features in Westeros apparently follow strictly Mendelian principles. According to The Lineages and Histories of the Great Houses of the Seven Kingdoms, Baratheon black hair has been winning out against Lannister blonde for generations.

In the years since the first season, it has become common knowledge that Joffrey (RIP), Myrcella (RIP?), and Tommen Baratheon are adorable little twincest babies, which is why they take after their golden-haired father/uncle as much as their mother/aunt.

Joffrey's Coat of Arms. Credit: TenTonParasol/Eigenes Werk

But are the rules of heredity in Game of Thrones always so straightforward? Alas, no. Take one of the most ill-fated characters in the show: Princess Shireen of House Baratheon, who Stannis proudly claims as his daughter (just wanted to twist that knife a little there). But if Shireen is Stannis's daughter, why didn't she inherit his black hair?

Chart illustrating recessive and dominant genes in shrimp. Credit: J C D

To my knowledge, Shireen's hair color was never confirmed in the books, but Kerry Ingram, the actress who portrayed her in the series, is most definitely fair-haired. She doesn't even have a Lannister parent to blame her blonde hair on. Shireen's mother Selyse is—or, was—a brunette from House Florent.

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Of course, the most likely explanation here is that the showrunners simply forgot that the seed is supposedly strong, and cast Ingram without considering her hair color. But the discrepancy has nonetheless generated some entertaining tinfoil rumblings, including the notion that Shireen's greyscale somehow sucked the color from her hair, or that she was yet another bastard child posing as a Baratheon.

Shireen could also simply be the exception that proves the rule. In real life, children are sometimes born with drastically different features from their parents, so perhaps Shireen's hair is just a result of random genetic variation. But if so, it seems inconsistent for the entire plot of the first book and season to pivot around the discovery that Baratheon children are unfailingly noirettes, only to later introduce a Baratheon child who is inexplicably blonde.

Along those lines, Shireen isn't the only character whose unusual inherited traits expose the mysterious hereditary logic of Game of Thrones. Daenerys Targaryen is also an odd case study, given that she seems to have a genetic resistance to heat and flames, which has passed through her Valyrian fire-and-bloodline.

George R. R. Martin has publicly gone on record to clarify that Daenerys is not actually flame retardant, and that her miraculous survival on Khal Drogo's funeral pyre in season one was just a one-time deal. "Targaryens are not immune to fire!" he said. "The birth of Dany's dragons was unique, magical, wondrous, a miracle. She is called The Unburnt because she walked into the flames and lived. But her brother sure as hell wasn't immune to that molten gold."

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That said, the first season depicts Dany walking into scalding hot baths and effortlessly touching objects that burn her Dothraki servants, so the message is a little muddled. Add to that the fact that the book version of last week's dragon-ex-machina moment has Drogon fire-belching right into her face with breath "hot enough to blister skin," yet she receives only minor burns.

Does this mean Targaryens have varying degrees of flame resistance? My bet is that there is some kind of genetic component to the Targaryen ability to withstand heat, but that it is recessive and it doesn't protect Targaryens from extreme heat sources, like dragonfire. Because the Targaryens have so many other otherworldly inherited features—such as silver hair and purple eyes—it seems reasonable that they might also have some degree of heat resistance imprinted in their DNA.

Game of Thrones season one recap. Credit: GameOfThrones/YouTube

On the flip side of the Braavosi coin, Starks may have a genetic resistance to the cold, plus they also have a unique family tradition of mindmelding with other creatures. Though the Starks aren't the only characters who are able to warg into animals—or in Bran's case, into humans—the strong tradition of skinchanging in the North suggests that there may be an wider genetic predisposition to warging abilities among the Northern population.

The mind-melds between the Starks and their direwolves, for instance, are especially strong, suggesting that there is, at least, some kind of symbiotic evolutionary relationship between Starks and wolves, mirroring the Targaryen partnership with dragons. Perhaps way back in Planetosi prehistory, there was some degree of interspecies gene exchange that imprinted atavistic wolf DNA in the Stark lineage and dragon DNA in the Targaryen lineage. Maybe these atavisms are now being activated by the upswing in magic across the realm.

After all, the viability of interspecies offspring is referenced as far back as the first season, when the wildling Osha catches an X-rated glimpse of naked Hodor. "He has giant's blood in him, or I'm the queen," she says. With that in mind, could Starks and Targaryens be descended from actual wolves and dragons, deeply rooted in their prehistory? Are their genomes imprinted with the evidence of this exchange, in the same way that Neanderthal DNA is imprinted into millions of humans today?

Moreover, could there be gene transfer between even odder interspecies couples, like humans and weirwood trees? These trees are certainly capable of mind-melding with humans, and the season four finale reveals that it's even possible to body-meld with them. So, in future seasons, will Maester Sam map Bran Stark's genome and find that it is some kind of unholy mess of arboreal, canine, and hominid alleles?

Probably not, but like so many other Game of Thrones mysteries, it is really fun to speculate about the finer details. The mechanisms governing genetic inheritance may have been blatantly spelled out in the early seasons, but at this point in the game, it looks like as if we are entering a post-Mendelian world.