The royal dynasty that ruled huge Spanish and Austrian territories over the last century was called the Habsburgs and were famous for more than their imperial majesty. The “Habsburg jaw”, a facial condition that afflicted these kings and queens, was well known. Now a new study says that this facial dysmorphism was due to inbreeding. The study was published in the Annals of Human Biology.
To arrive at this conclusion, the researchers looked at the portraits of these royal personages, from way back in history. They diagnosed the facial deformities and explored the degree of relationship between the royals, using genetic analysis. Their aim was to find a direct link, if any.
Portrait by Juan Carreño de Miranda, c. 1685, shows Charles's Habsburg jaw. King Charles II of Spain was the last in the Habsburg line and one of the most afflicted with the facial deformity. Credit: Don Juan Carreño de Miranda
Marriages of interest
The Habsburg dynasty built a vast empire across Europe, over more than 200 years, by force, intrigue – and intermarriage. Today, it is extinct as the last monarch, Charles II of Spain, could not produce even a single heir – the cost of long decades of inbreeding. Not only was he incapable of fathering children, but he was physically weak and short, and mentally incompetent, besides being often ill with digestive tract problems.
In fact, geneticists think the dynasty passed into oblivion because of their strong tendency to intermarry. Statistics show that only 50% of Habsburg children lived past the age of 10 years, while the average survival rate for other Spanish families at the time was more like 80%.
Apart from the Habsburg family, harmful recessive genes for hemophilia were spread across most of Europe’s royal families by this pernicious practice in Queen Victoria’s extended family. Recessive genes can cause disease only if both copies of the gene are defective, which is rare because most people have at least one healthy dominant copy, that masks any possible action of the recessive one. When family members intermarry, however, this dilution effect is absent. As a result, the risk that the offspring will inherit a defective copy of the gene from both parents, and thus develop the disease, is much higher.
The current study came about as a result of scientific curiosity to know whether the peculiar chin shape seen in these royals was because of the many intermarriages that led to the sharing of too many genes among too few. To explore this, 10 maxillofacial researchers evaluated 11 features of mandibular prognathism and 7 features of maxillary deficiency (small upper jaw, with the tip of the nose hanging over an excessively prominent lower lip). They were helped by the mostly realistic depiction of royalty in the 66 portraits of these rulers that they studied.
Mandibular prognathism is marked by lower jaw protrusion which hampers mouth closure if severe, but is always due to a significantly larger mandible, or lower jaw, compared to the upper jaw.
Scoring each Habsburg for both of these physical traits, the surgeons found that the least expression of both was in Mary of Burgundy, a 1477 entrant by marriage, while it was clearest in Philip IV, who ruled Spain and Portugal between 1621 and 1640. Taken singly, a small upper jaw was most strikingly present in five family members, namely Maximilian I, Margaret of Austria who was his daughter, Charles I who was his nephew, Philip IV who was Charles’ great-grandson, and Charles II.
Charles II, the last Habsburg, was unfortunate in being the child of his father and his father’s niece, making for a complicated family line. Not only were his parents very closely related, but they came from a long series of very closely related spouses, so that their mating was, as it were, the final blow. In fact, geneticists estimate that the inbreeding coefficient, or the likelihood that an individual will have two identical copies of the same gene because of related parents, for Charles II was almost the same as for a child born of incest. Incest refers to mating between siblings, or first-order relatives.
Charles II was painted with the famous jawline accurately represented. While this feature, which is called mandibular prognathism, is the most striking physical trait of this family, inbreeding occurred at the cost of many others as well.
The genetic analysis was made using a family tree that went back 20 generations and showed over 6000 people. The degree to which inbreeding was present was clear from this tree. This was then studied to decide if it was the cause of the deformity.
Not only were both conditions seen to a greater-than-normal extent in this family, but there is a correlation between their occurrence, showing that both together make up the Habsburg jaw deformity, and that both are determined by the same set of genes.
The researchers found that the greater the number of consanguineous marriages, the greater the degree of mandibular prognathism, to a level of statistical significance. Maxillary deficiency is also linked to the degree of inbreeding, but the connection was significant in only 2/7 of the diagnostic features.
Researcher Roman Vilas says, “We show for the first time that there is a clear positive relationship between inbreeding and appearance of the Habsburg jaw.”
Why these features are related remains to be established. However, the scientists think it is probably because of the increased chances of inheriting the same form of a given gene from both parents, making the offspring homozygous for that gene. Homozygosity is generally associated with unfitness on a genetic level, and this makes the famous Habsburg jaw a marker of intrinsic genetic weakness rather than strength.
The scientists are unwilling to rule out the occurrence of genetic drift as the reason, or chance appearance of the mandibular prognathism trait, in view of the small number of people involved in the genetic analysis. However, they concede that these are unlikely explanations.
The Habsburgs would be outraged to hear themselves described as “a human laboratory”, in Vilas’ words, but the scientist treats this inbreeding family as such, to help understand how this practice affects genetic and physical fitness. Consanguineous marriages are still commonly practiced today in many parts of the world and in some people groups, on religious or cultural grounds. This study provides an initial scary insight into the eventual outcome of such a practice.
Román Vilas, Francisco C. Ceballos, Laila Al-Soufi, Raúl González-García, Carlos Moreno, Manuel Moreno, Laura Villanueva, Luis Ruiz, Jesús Mateos, David González, Jennifer Ruiz, Aitor Cinza, Florencio Monje & Gonzalo Álvarez (2019) Is the “Habsburg jaw” related to inbreeding?, Annals of Human Biology, DOI: 10.1080/03014460.2019.1687752, https://www.tandfonline.com/doi/full/10.1080/03014460.2019.1687752