Game of Thrones’ audience has known that in indulging in a relationship with Daenerys Targaryen, Jon Snow was inadvertently sleeping with his aunt. In Sunday’s premiere, Jon himself learned of his true parentage, which presumably means that, somewhere off screen clad in furs and looking contemplative, he too connected the dots about his new fling. We all know that in the world of Westeros, twin siblings have procreated without genetic repercussions, but how bad, exactly, is a nephew-aunt relationship?
Scientifically speaking, two people with overlapping genetic material increases the likelihood that their offspring will inherit two faulty copies of a gene and therefore experience a disorder, geneticist Tiong Tan writes in the Conversation: “For many genes, our body can cope with just a single working copy, but when both copies are faulty, the person gets an autosomal recessive disease,” like sickle cell anemia or cystic fibrosis. Even still, cousin-cousin couplings are common in many societies and have been throughout history—Einstein and Darwin both did it. And while the risk of genetic issues roughly doubles, the absolute risk is still very low (somewhere between 4 and 7 percent), meaning that the vast majority of babies from such unions will still be healthy. After evaluating the risk, Popular Science declared, “Go ahead, marry your cousin.”
If cousin marriage is technically OK, how much does the risk increase with a closer genetic link? As Tan explains, while a pair of first cousins share just an eighth of their genes, parents and kids share half their genes, and an aunt and a nephew share a quarter. A 1971 study estimated that it’s four times as genetically risky for parents and kids to have offspring vs. first cousins, as the coefficient of inbreeding—a measure of genetic closeness—for the kid is one-fourth versus one-sixteenth. Extrapolating from there, an aunt-nephew baby would have an inbreeding coefficient of one-eighth, meaning reproduction would carry double the genetic risk compared with a cousin-cousin baby. Not great. (Another lesson: don’t google “aunt incest” at work. That yields porn.)
In modern society, what’s more problematic than the risk of disease (present in many pairings that don’t involve folks who are related) and even the substantial taboo (we’ve come around on a number of sexual acts and couplings that were once taboo) are the interpersonal realities. In 2014, a New York court ruled that an uncle-niece marriage was allowed (New York is one of 20 states to also allow cousin-cousin marriage), partially on the grounds that the uncle in question was only half-siblings with the bride’s mother. But as one judge pointed out, incest laws don’t exist just to keep the likelihood of genetic disease down—they exist because of the deep power imbalances that can exist between older and younger family members.
Even with cousins who are closer in age, the logistics are tricky. As Slate’s Dear Prudence, Daniel Ortberg, counseled one man who wanted to sleep with his first cousin, what happens if things don’t work out and then you have to keep running into each other at family gatherings? The previous Prudie ruled in the opposite direction in a separate case—when Emily Yoffe got the cousin question, there was an additional twist in that the two partners were unaware of their genetic links. Yoffe counseled the letter writer—who secretly fathered one of the kids via an affair—to stay quiet and let the couple be.
Jon and Dany find themselves in different circumstances, of course. The Targaryens aren’t exactly getting together for holidays. Dany can’t bear (human) children. Still, the familial connection might still cause trouble if Snow’s parentage is more widely learned—it’s clear that at least some forms of incest are taboo in Westeros.
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