Credit: Game of Thrones / HBO.

During Game of Thrones’ previous season finale, viewers had the confirmation of Jon Snow’s true identity. As many die-hard fans of George RR Martin’s books have known, or at least suspected, for years, Jon Snow’s father is Rhaegar Targaryen, the young prince who secretly married Jon’s mother and Ned Stark’s sister Lyanna in the Tower of Joy.

There’s a big dilemma with this newfound insight, though. You see, it means that Jon Snow, or Aegon Targaryen by his birth name, is Daenerys Targaryen’s nephew. The two are now lovers which can only mean there’s a big risk any children the two might birth would be extremely disabled or messed up in some way.

A game of chance?

According to Jonathan Pettitt, a University of Aberdeen geneticist who actually calculated what are the odds of the baby being born with problems, the pair share 44 to 47.5 percent of their DNA. Siblings typically share 50 percent of their DNA while unrelated people ought to share zero.

As such, any child produced in this incestual union shouldn’t be healthy by our universe’s standards.

“Jon Snow is not so inbred as Dany, so their offspring would be less inbred. The inbreeding coefficient of any child of Jon and Daenerys would have an inbreeding coefficient (calculated as half of their relationship coefficient) of 22 percent. So, slightly less than that of Charles II of Spain, though not by very much,” Pettitt told Inverse. 

Targaryen family tree. Credit: HBO

The fact that Jon is only half Targaryen softens things up a tad but Daenerys still has an extremely high coefficient of inbreeding of 37.5 percent since both her parents and her grandparents were siblings. She didn’t turn out too bad, of course, but unless genetics in R.R. Martin’s universe works fundamentally different, we can consider she was plain lucky.

Painting of Charles II of Spain (6 November 1661 – 1 November 1700).

Take Charles II of Spain, for instance. He was the last king of the Spanish Hapsburgs line, a dynasty where uncle-niece, first cousins and other consanguineous unions were prevalent. He died prematurely aged 39 but not before his imbecile behavior plunged his kingdom into chaos eventually leading to the War of Spanish Succession. This was the first world war of modern times with theatres of war in Spain, Italy, Germany, Holland, and at sea. It’s estimated the war resulted in 400,000 casualties. 

Here’s how one biography describes King Charles (Carlos) II:

“The Habsburg King Carlos II of Spain was sadly degenerated with an enormous misshapen head. His Habsburg jaw stood so much out that his two rows of teeth could not meet; he was unable to chew. His tongue was so large that he was barely able to speak. His intellect was similarly disabled. His brief life consisted chiefly of a passage from prolonged infancy to premature senility. Carlos’ family was anxious only to prolong his days and thought little about his education, so that he could barely read or write. He had been fed by wet nurses until the age of 5 or 6 and was not allowed to walk until almost fully grown. Even then, he was unable to walk properly, because his legs would not support him and he fell several times. His body remained that of an invalid child. The nature of his upbringing, the inadequacy of his education, the stiff etiquette of his court, his dependence upon his mother and his superstition helped to create a mentally retarded and hypersensitive monarch.”

Like Daenerys, Charles II was the culmination of generations of cousin and sibling marriages. Fortunately, the beautiful silver-haired Daenerys Stormborn of the House Targaryen, First of Her Name, the Unburnt, Queen of the Andals and the First Men, Khaleesi of the Great Grass Sea, Breaker of Chains, and Mother of Dragons, never inherited the famous Habsburg jaw.

We can’t rule out though that Charles’ faith won’t befall any offspring Dany and Jon have.

“They would still be predicted to be fairly sick based on real-world genetics,” says Pettitt. “So, overall, not likely to be great material for founding a future dynasty!”