‘Game Of Thrones’ Incest Could Be Worse, Relatively, Than Aunt-Nephew Relations, Genetically Speaking

Emilia Clarke as Daenerys Targaryen in Game of Thrones
Helen Sloan / HBO

Incest has been a theme on Game of Thrones since literally the first episode, and recent episodes have revealed that two characters, aunt and nephew, are themselves in an incestuous relationship. Is such a relationship rolling the dice genetically? Probably not, a geneticist says via Slate.

WARNING: The remainder of this post contains open spoilers for Game of Thrones.

Western society, culture, and law have, for the past century or so anyway, regarded marriage and sexual relationships between “close” relatives as taboo for a variety of reasons. Outside of the West, however, cousin-cousin marriages aren’t particularly uncommon. And indeed, even in the West, they weren’t all that uncommon even a few decades ago; Einstein and Darwin both kept it in the family, so to speak, marrying their cousins and having perfectly normal kids with them. And indeed, most living European royals are no more than sixth-or-so cousins from each other; Queen Elizabeth and her husband, Prince Philip, are third cousins, for example, according to Business Insider.

But Jon Snow and Daenerys Targaryen’s relationship is a bit closer than sixth or third or even first cousins: they’re nephew and aunt, respectively.

So is that a bad move, genetically speaking? Are an aunt and nephew who conceive a child at risk of producing one with birth defects?

Excuse the pun, but it’s all relative, says geneticist Tiong Tan. In essence, aunt-nephew incest is a much safer proposition, genetically speaking, than sibling-sibling or parent-child incest. But it’s riskier than cousin-cousin incest, as he explains in The Conversation.

“Most of us carry a handful or so of faulty recessive genes so [producing children] within your family increases your chance of [pairing with] someone else with the same faulty recessive genes as you.”

Putting it in more concrete terms, a 1971 paper hosted on Karger Publications discusses what’s called the “coefficient of inbreeding,” a sort of mathematical look at the risk of passing on birth defects through close marriages based on the number of overlapping genes the couple share. Without getting into the science of it, larger numbers are worse. Here’s how they break down:

Identical twins: one-to-one.
Non-twin siblings: one-half.
Parent-child: one-fourth.
Aunt-nephew (or uncle-niece): one-eighth.
Cousin-cousin: one-sixteenth.

In terms of absolute risk, the odds of a child being born with birth defects from a cousin-cousin relationship are between 4 and 7 percent, and presumably higher in an aunt-nephew relationship.

These days, such a relationship would be taboo and probably illegal. Only 20 of the 50 states allow cousin-cousin marriages, and “avunculate marriages” (between aunts and nephews or uncles and nieces) are so rare that it generally takes a court decision to see if it’s even allowed. In 2014, a New York judge signed off on a marriage between an uncle and a niece, but the uncle had a different father than the niece’s mother, as The New York Daily News reported at the time.

So, long story short: Jon and Daenerys can do as they please, since incest in their universe’s monarchy has been a thing for centuries, and one of them, anyway, is the true heir to the throne. And genetically speaking, the risk of them having a baby with birth defects is comparatively low.