Devastating inbreeding led to incurable defect in royal family

Irish Daily Mirror

09-05-2025

Once a mighty Royal dynasty at the helm of sprawling Spanish and Austrian empires, the Habsburgs are infamous nowadays for the pronounced "Habsburg jaw", a dramatic facial deformity. According to one study, inbreeding is the culprit behind this peculiar trait.
Scientists pored over historical portraits to dissect the distinct facial characteristics of the Habsburg royals, connecting the dots between their physical traits and genetic backgrounds, aiming to confirm if inbreeding caused these features. The investigation pointedly notes that Charles II of Spain, the last effectual Habsburg sovereign, grappled with a myriad of health issues, including infertility—which was likely a side effect of extensive inbreeding through the generations.
The various ailments of Charles II inhibiting his ability to sire an heir led to the ultimate extinction of the Habsburg reign. Geneticists point the finger at the incestuous practices of the dynasty, emphasising the harrowing statistic that merely half of the Habsburg offspring survived beyond their tenth year, a sharp discrepancy from the 80% survival rate amongst other Spanish progenies of that era.
Not exclusive to the Habsburgs, Europe's Royal bloodlines elsewhere also suffered the spread of pernicious recessive genes linked to haemophilia due to the widespread intermarriage within Queen Victoria's vast progeny. These recessive genes normally only unleash illness when individuals inherit defective versions from both parents, which, thankfully, is infrequent since most carry a robust dominant gene to suppress the said recessive defect.
However, when relatives wed, this dilution effect is missing, heightening the chance of children inheriting a faulty gene from both parents and consequently developing the disease. This study sought to determine if the unique chin shape observed in these royals was a result of excessive intermarriage leading to an overabundance of shared genes, reports the Express.
To delve into this, a team of 10 facial researchers scrutinised 11 characteristics of the chin feature known as mandibular prognathism, along with seven features of maxillary deficiency (a small upper jaw, with the nose tip hanging over a notably prominent lower lip), using the most accurate depictions available in the 66 portraits of these monarchs they studied.
Mandibular prognathism is marked by a jutting lower jaw which can interfere with mouth closure if severe, but is invariably due to a significantly larger mandible, or lower jaw, compared to the upper jaw.
The surgeons rated each Habsburg for these physical traits and found the least expression in Mary of Burgundy, who became part of the family through marriage in 1477, while it was most noticeable in Philip IV, who reigned over Spain and Portugal from 1621 to 1640. A small upper jaw was notably present in five family members: Maximilian I, his daughter Margaret of Austria, his nephew Charles I, Charles' great-grandson Philip IV, and Charles II.
Charles II, the last Habsburg, was born to his father and his father's niece, resulting in a complicated family tree. His parents were not only closely related but also descended from a long line of closely related couples, making their union, as it were, the final blow.
Geneticists estimate that Charles II was medically on a par with an inbred person. As the probability of having two identical copies of the same gene due to related parents, was nearly equivalent to a child born of incest, which refers to mating between siblings or first-order relatives.
Charles II's famous protruding jawline, known as mandibular prognathism, was not just an aesthetic hallmark of his lineage but also a clear sign of the extensive inbreeding that took a toll on the family's health. Geneticists delved into a family tree covering 20 generations, showcasing more than 6,000 individuals, which illuminated the depth of inbreeding among the Habsburgs.
This genealogy laid the groundwork for research into whether inbreeding caused the notorious facial deformity. Findings confirmed that both the Habsburg jaw and the related maxillary deficiency were significantly more common within this inbred group, suggesting a genetic basis for this distinct trait.
The study found a significant positive correlation between inbreeding and the severity of mandibular prognathism; however, maxillary deficiency's link to inbreeding only held statistical significance in two out of seven diagnostic features examined.
Researcher Roman Vilas sums up the findings: "We show for the first time that there is a clear positive relationship between inbreeding and appearance of the Habsburg jaw."
While the exact link between these traits remains under investigation, researchers suggest that it likely stems from the higher chance of inheriting identical gene variants from both parents, leading to homozygosity at particular genes.
Such genetic homogeneity often correlates with reduced genetic fitness, casting the notorious Habsburg jaw as an indicator of inherent genetic weakness rather than strength.
The scientific community remains cautious about ruling out genetic drift or the spontaneous occurrence of the mandibular prognathism characteristic, considering the small pool of individuals in the genetic study. Still, they concede that these alternative explanations seem unlikely.
To know the Habsburgs by the ignominious title of "a human laboratory" might well have outraged them, yet Vilas uses this term in context to emphasize how this dynasty serves as a model to examine the repercussions of inbreeding on genetic robustness and physical health.