The Royal House of Habsburg, one of the most powerful dynasties of Medieval and Renaissance Europe, reigned over much of Europe for centuries, and a lesson in genetic inheritance may explain their demise.
Beginning in the early 12th century they quickly expanded their realm. They did this through a series of strategically executed marriages, from the mountains of Switzerland to a territory that included swaths of Austria, Hungary, Italy, France and Spain. The Spanish branch of the Habsburg dynasty helped create an empire that reached its apex in the 16th and 17th centuries. They controlled land from the Phillippines to the Americas.
Yet the Habsburgs are known not only for controlling huge tracts of Europe, but also for maintaining control by rarely marrying outside the dynasty.
By the end of the 17th century, the results of their marital practices had become apparent. Family members had distinctive protruding lips, a high rate of infant mortality and a host of other health problems. Researchers in 2019 found that the distinctive jutting “Hapbsburg Jaw” was likely the result of inbreeding.
Could the same marital practices that helped bring the Habsburg dynasty to power also have led to its demise?
In PLoS One, scientists from Spain’s University of Santiago de Compostela argue that it did. They said that inbreeding so incapacitated the Habsburgs that by the death of King Charles II of Spain in 1700, they were virtually unable to reproduce.
From 1516 to 1700, it has been estimated that over 80% of marriages within the Spanish branch of the Habsburg dynasty were consanguineous. In other words they were marriages between close blood relatives. Most often, these unions took the form of marriages between first cousins, double-first cousins, and uncles/nieces. Infant and child mortality rose to 50% among Spanish Habsburgs, much higher than the average for the period.
Habsburg Empire Threatend
But the final Habsburg king of Spain, Charles II, was perhaps the most unfortunate result of these unions. Also know as “El Hechizado” (“The Hexed”), Charles was severely deformed. The so-called “Habsburg Lip”, a form of mandibular prognathism often seen among members of the Habsburg Dynasty, was so pronounced in Charles’ case. So much so that it was difficult for him to speak. An enlarged tongue, gastrointestinal problems, mental retardation, and possible growth problems meant that Charles was raised almost as an infant until the age of 10. Even as he grew older, he was never able to govern effectively. His rule saw the rapid decline of the Empire, only exacerbated by his death in 1700.
But for all the speculation and anecdotal evidence of the negative impact of inbreeding on the House of Habsburg, there has been little scientific research. So there is not hard evidence whether inbreeding actually played a role the demise of the Hapsburg line. The authors of this study sought to achieve this goal by examining genealogical information. To do that they looked at family pedigrees, for the eight royal families connected with the Habsburg dynasty.
A Long Pedigree
All told they analyzed family pedigrees of over 3,000 individuals spanning 16 generations. They then used this information to calculate the inbreeding coefficient for each family member. The inbreeding coefficient is simply a measure of the chance that someone will receive an identical set of genes from both parents.
Unsurprisingly, the authors found elevated inbreeding coefficients for many Habsburgs. In fact, the levels increase consistently from the earliest Spanish Habsburgs. This goes from King Philip I (1478-1506), to Charles II, the last Spanish Habsburg king. Some of the Habsburgs – notably Charles II – had an inbreeding coefficient nearly twice what one would expect given the level of relatedness between his mother and father. In other words, even though Charles’ parents were related to each other as uncle and niece, his inbreeding coefficient fell at the same level as someone whose parents were brother and sister.
These unexpectedly high levels indicate that consanguineous marriages, like that of Charles’ parents. These unions had probably been happening along the Habsburg line for hundreds of years. This practice came to a head with the birth of Charles, whose inbreeding coefficient was the highest of all the Spanish Habsburgs. His physical deformities were also the most severe.
Further, the authors argue that Charles’ ill health was a direct result of centuries of consanguineous unions. Specifically, they point to a growth hormone deficiency and severe renal tubular acidosis. These may have accounted for his short stature and his many physical deformities and ailments. These diseases are quite rare in the general population. But the fact that so many of Charles’ ancestors were related to each other would have increased his chances of inheriting the genes associated with them. Whether Charles did in fact suffer from these specific diseases is still open to interpretation. However, it is clear that his physical and mental difficulties prevented him from fathering any heirs to the throne.
The Habsburg dynasty in Spain ended when Charles passed away in 1700. He was a few days shy of his 39th birthday.