Incurable facial defect in royal family caused by 'too much inbreeding'

The Habsburgs, once the powerhouse behind expansive Spanish and Austrian empires, have become synonymous with the infamous "Habsburg jaw", a severe facial deformation attributed by some to excessive inbreeding within the dynasty.

A study closely analysed historical portraits of the family to investigate their distinctive facial features, pairing this visual assessment with genetic analysis to unravel whether these traits were indeed the results of intermarriage. The investigation highlights that the last Habsburg monarch, Charles II of Spain, was plagued by various health issues including infertility - which may have stemmed from centuries of the dynasty keeping it in the family, so to speak.

Such genetic entanglements resulted in dire consequences for the Habsburg reign, as Charles II's debilitating conditions left him without a successor, bringing an abrupt halt to the Habsburg legacy. Experts point out the severity of their inbreeding practice by revealing that only half of the offspring survived past the age of 10, a stark discrepancy from the 80% survival rate in other Spanish noble families during that period.

READ MORE:

READ MORE:

Moreover, the European royals faced the rampant spread of haemophilia, often directly traced back to Queen Victoria's lineage, exemplifying another peril of Royal intermarriage. Recessive genes usually remain innocuous unless a matching pair is present—an unlikely circumstance given that most people carry at least one dominant gene to counterbalance any recessive defects.

However, when relatives tie the knot, this dilution effect is missing, heightening the likelihood of their 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 .

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 an overly prominent lower lip), using the most accurate depictions available in the 66 portraits of these rulers 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 convoluted 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 believe that medically, Charles II was equivalent to an inbred person. The likelihood of him having two identical copies of the same gene due to his parents' close relation was nearly the same as a child born of incest, which refers to mating between siblings or first-order relatives.

Charles II's infamous protruding jawline, known as mandibular prognathism, was not just a distinctive feature of his lineage but also a clear indication of the extensive inbreeding that impacted the family's health. Geneticists explored a family tree spanning 20 generations, featuring more than 6,000 individuals, which highlighted the depth of inbreeding among the Habsburgs.

This genealogy set the stage for research into whether inbreeding caused the notorious facial deformity. The 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 unique trait.

The study found a significant positive correlation between inbreeding and the severity of mandibular prognathism; however, the link between maxillary deficiency and inbreeding only held statistical significance in two out of seven diagnostic features examined.

Roman Vilas, the lead researcher, encapsulated the findings by saying, "We show for the first time that there is a clear positive relationship between inbreeding and appearance of the Habsburg jaw."

While science is still unravelling the exact link between such traits, it's thought to likely stem from the greater chance of receiving identical gene variants from both parents – this kind of genetic match-up can result in homozygosity.

A tell-tale sign associated with such genetic matching is often "genetic unfitness", which means the notorious Habsburg jaw might actually symbolise underlying genetic vulnerability as opposed to regal might.

Despite the current understanding, experts remain cautious about completely ruling out genetic drift or the random appearance of mandibular prognathism due to the limited sample size of the study. Nevertheless, these other causes are considered less likely.

Revering themselves as anything but ordinary, it would no doubt ruffle the feathers of the Habsburgs to be described as "a human laboratory", a term used by Vilas. Yet, the scientist calls upon this uniquely inbred dynasty as just that – seeking insights into the effects of inbreeding on one’s genes and health.