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Cersei and Jamie Lannister, from Game of Thrones, were madly in love. They fiercely fornicated, passionately protected each other and birthed multiple children.

They were also brother and sister.

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But while the fantasy show also includes fire-breathing dragons and the frozen undead, inbreeding in royal families is far from fiction. In fact, a new study reveals it played a part in creating one of history’s ugliest rulers.

Researchers from Spain and South Africa say inbreeding is strongly correlated to the “Habsburg jaw,” a distinctive condition in the Habsburg dynasty of Spanish and Austrian families characterized by an oversized jaw, a large lower lip and a hanging nose tip.

(The researchers define inbreeding as having children with anyone closer than a second cousin.)

Photo by Wikimedia Commons

The team looked at historical portraits of royal families, focusing on the Habsburg dynasty. King Charles II of Spain was the bloodline’s final descendant and “one of the most afflicted.” The study says his distinct feature and inability to produce an heir show how inbreeding and incest impact genetics.

Researchers recruited 10 maxillofacial surgeons to look at portraits of the family and rate them on the severity of 18 dysmorphic features.

Each family member received a score for the prominence of their features. Then, using those scores and genetic analysis, the research team calculated an “inbreeding coefficient,” to determine the role incest played in creating the facial features.

The scores were also adjusted by sex since men of the Habsburg family were more severely affected by the Habsburg jaw than women.

And this study was the first to show the infamous chin was strongly correlated to a bloodline brimming with kissing cousins and spit-swapping siblings.

“Don’t get married with your sister or your mother,” says Francisco Ceballos, one of the study’s authors in a phone interview with the National Post. “Huge amounts of inbreeding is going to be bad and you’re going to be ugly as f-ck like Charles the II.”

Don’t get married with your sister or your mother

Incest was a common tactic among royal families to retain power.

For the Habsburgs, intermarriage and “a huge amount of inbreeding” protected the family’s influence as successive generations occupied the throne of the Holy Roman Empire between the 1400s and 1700s.

The study found that more than 6,000 individuals belonged to only about 20 sets of parents.