The Habsburgs, once a powerful Royal dynasty ruling vast Spanish and Austrian empires, are now infamous for the pronounced “Habsburg jaw”, a stark facial deformity. One study suggests that this distinct feature was caused by inbreeding.
Researchers examined historical portraits of these monarchs to study their unusual facial quirks and trace familial ties through genetic analysis, attempting to establish whether or not their features were in fact the product of inbreeding. Indeed, the last ruler, Charles II of Spain, suffered from multiple illnesses including infertility—likely due to generations of inbreeding .
Charles’ physical and mental weaknesses meant he failed to produce an heir, ending the Habsburg lineage. Genetic researchers argue that this downward spiral stemmed from their incestuous activities; as evidence, they note that only half of Habsburg offspring survived past a decade, a striking contrast to the 80% average among other Spanish families at the time.
Aside from the Habsburg family, harmful recessive genes for haemophilia were spread across most of Europe’s Royal families due to intermarriage within Queen Victoria’s extended family. Recessive genes can only cause disease if both copies are defective, which is rare as most people have at least one healthy dominant copy that masks any possible action of the recessive one.
However, when family members marry each other, this dilution effect is absent, increasing the risk of offspring inheriting a defective gene from both parents and developing the disease. This study aimed to find whether the distinctive chin shape seen in these royals was due to the many intermarriages leading to too much gene sharing among too few.
To investigate this, 10 facial researchers examined 11 features of the chin feature, called mandibular prognathism, and seven features of maxillary deficiency (small upper jaw, with the tip of the nose hanging over an excessively prominent lower lip), using the mostly realistic depiction of royalty in the 66 portraits of these rulers they studied.
Mandibular prognathism is characterised by a protruding lower jaw which can hinder mouth closure if severe, but is always due to a significantly larger mandible, or lower jaw, compared to the upper jaw.
The surgeons scored each Habsburg for these physical traits and found the least expression in Mary of Burgundy, who joined the family by marriage in 1477, while it was most evident in Philip IV, who ruled 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 complex 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 summed it up by stating, “We show for the first time that there is a clear positive relationship between inbreeding and appearance of the Habsburg jaw.”
The connection between these characteristics is yet to be determined. However, scientists believe it’s likely due to the increased probability of inheriting the same gene variant from both parents, resulting in homozygosity for that gene.
This genetic uniformity is typically linked with genetic unfitness, making the infamous Habsburg jaw a sign of inherent genetic frailty rather than power.
Scientists are hesitant to dismiss the possibility of genetic drift or the random emergence of the mandibular prognathism trait, given the limited number of individuals involved in the genetic analysis. Yet, they admit these are improbable explanations.
The Habsburgs would be incensed to hear themselves referred to as “a human laboratory”, in Vilas’ words, but the scientist uses this inbred family as such to explore the impact of inbreeding on genetic and physical health.
