(Original Caption) Queen Elizabeth II and Prince Phillip. She is seated and wearing a crown, he is standing in uniform. Undated photograph.
We Finally Know Why This Royal Family Shared A Distinct Facial Feature
Lifestyle - News
As it turned out, Queen Elizabeth and Prince Phillip were third cousins, both descended through Queen Victoria. Relative weddings have been practiced since the 1500s; in fact, research suggests that a particular physical trait connected to a former royal line may have originated through cousin marriages.
If you were to look at portraits of royals like Philip IV and Charles II of Spain, who were a part of the Habsburg dynasty and ruled in the 17th century, your eyes will most likely be drawn to their protruding chins. That facial feature, which can be seen in portraits, was shared amongst kings and other members of the royal family.
In 2019, "Annals of Human Biology" published a study looking at 15 people with Habsburg ancestry, which changed the direction of this discussion in terms of research. The study set out to see if inbreeding in the Spanish family was caused by strong, hereditary facial traits, and after dissecting the findings, it seems likely that inbreeding is a reason for the "Habsburg jaw."
Genetics professor Roman Vilas from the University of Santiago de Compostela confirms this with the findings of his study, saying in a statement: "We show for the first time that there is a clear positive relationship between inbreeding and appearance of the Habsburg jaw." Inbreeding is believed to be the demise of the Habsburg dynasty.