The condition, medically known as mandibular prognathism, causes the lower jaw to protrude significantly and affected the Habsburg dynasty of Spanish and Austrian kings and their wives, who secured their influence across a vast swathe of Europe for more than 200 years through intermarriage.
While these genetically compromising tendencies are known to have eventually led to the family’s demise, with the final Habsburg monarch, Charles II, unable to produce an heir as a result, no studies had conclusively linked the distinctive facial condition to their inbreeding.
“The Habsburg dynasty was one of the most influential in Europe, but became renowned for inbreeding, which was its eventual downfall,” said the study’s lead researcher, Professor Roman Vilas, from the University of Santiago de Compostela.
Habsburg descendants ruled over the Holy Roman Empire, Austria, and in Spain until the heirless death of Charles II, also known as “the Bewitched”, which led to the 13-year War of the Spanish Succession in 1701.
“We show for the first time that there is a clear, positive relationship between inbreeding and appearance of the Habsburg jaw,” Professor Vilas said.
“While our study is based on historical figures, inbreeding is still common in some geographical regions and among some religious and ethnic groups, so it’s important today to investigate the effects.
“The Habsburg dynasty serves as a kind of human laboratory for researchers to do so, because the range of inbreeding is so high.”
During the study, published in the Annals of Human Biology, researchers recruited 10 specialist facial surgeons, who used 66 portraits to diagnose the condition in 15 members of the Habsburg dynasty.
Despite differences in artistic style, the portraits are characterised by a realistic approach to the human face.
The surgeons were asked to diagnose 11 features of mandibular prognathism, as well as seven features of maxillary deficiency, the most recognisable of which are a prominent lower lip and an overhanging nasal tip.
The portraits are held by some of the world’s most esteemed art museums, including the Kunsthistorisches Museum in Vienna and Madrid’s Prado Museum.
The surgeons gave scores for the degree of mandibular prognathism and maxillary deficiency in each member of the Habsburg family.
Mary of Burgundy, who married into the family in 1477, showed the least degree of both traits. The Habsburg jaw was most pronounced in Philip IV, who was king of Spain until 1665.
Maxillary deficiency was diagnosed to the greatest degree in five members of the family, including Charles II.
Detecting a correlation between the two conditions, the researchers speculated that they share a genetic basis.
The extent of inbreeding was calculated from a large-scale family tree, including more than 6,000 individuals belonging to more than 20 generations.
The researchers detected a strong relationship between the degree of inbreeding and the extent of the Habsburg jaw. The relationship to maxillary deficiency was also positive, but it was only statistically significant in two of the seven features diagnosed.
The cause of the relationship between the inbreeding and facial deformity remains unclear, but the study's authors suggest it is because mating between relatives increases the chance of offspring inheriting identical forms of a gene from both parents.
This reduces someone’s genetic fitness – meaning the Habsburg jaw should be considered a recessive condition.
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