Long before the concept of "designer babies" created in a lab became the stuff of science fiction, inbreeding in royal families was viewed as a way to ensure genetic purity. Intermarriage ensured that no "common" blood sullied pure, aristocratic bloodlines. What could go wrong?
A lot, actually. Birth defects caused by inbreeding were rampant in royal families from Russia to Portugal and even in ancient Egypt, where the practice of sibling marriage was considered godly behavior. Hereditary diseases caused by inbreeding - such as porphyria, among others - get handed down through thin gene pools, particularly in the many cases where intentional close marriage is used to ensure that royal blood (and its recurrent flaws) are kept in the family. For example, Queen Victoria, a major proponent of pure bloodlines, married her cousin Albert, and the two had nine children who then passed hemophilia to royal families throughout Europe.
While all these families hoped close intermarriage would keep their royal families stronger, in many cases, illness, madness, and infertility caused by inbreeding ended up tearing them apart.
The product of a long line of Habsburg inbreeding, Charles II (nicknamed "The Bewitched") looked the part. He had what was called the Habsburg Jaw or Habsburg Lip, characterized by a huge tongue, an under-bite, a jutting lower jaw, and a thick lower lip. Technically, the deformity is known as mandibular prognathism. His tongue made it difficult to chew and caused excessive drooling.
The king was also severely developmentally delayed. He was breastfed until he was 5 and never received any formal education. He was also impotent, so his inability to procreate ended the Habsburg’s hold on the Spanish crown when the king passed in 1700.
The Habsburg dynasty had been intermarrying for so long that one of Charles's ancestors, Joanna of Castille, appears in his family tree 14 different times. In fact, Charles I was more inbred than he would have been if his parents had been brother and sister.
Tsarevich Alexei Nikolaevich's hemophilia contributed to the fall of the Russian imperial dynasty in 1917. While no one in the ruling Russian House of Romanov was known to carry hemophilia (a potentially fatal genetic disease where blood does not clot normally), Alexei's father Tsar Nicholas had married into the family of Queen Victoria of England, who was part of a clan of passionate inbreds. Tsarist Alexandra was Victoria's granddaughter.
Desperate to save her son's life, Alexandra sought mystical intervention in the form of Rasputin, the "Mad Monk.” Inviting a lascivious man like Rasputin, known for his tastes for alcohol and lovers of all genders, didn’t go over well with the aristocracy. The Russian rulers, however, were convinced that Rasputin’s treatments were effective in saving their son.
The mystic Rasputin gained greater influence at court, wielding increasing power over the lives of the royals and surreptitiously governing Russia. The disorder in the royal house and the questionable company the rulers kept helped spur the Russian Revolution of 1917, and as a result, the entire royal family was executed.
Although his legacy is as the golden boy pharaoh of ancient Egypt, DNA tests of King Tut's mummified corpse show that this ruler of Egypt circa 1300 BCE was actually a feeble-bodied genetic misfit, owing to the Egyptian royal tradition of brothers and sisters marrying one another. King Tutankhamun took the throne at age 9 and survived only until the age of 19. He likely had a cleft palate, a club foot, and scoliosis, as well as an elongated, deformed skull.
Egyptian pharaohs revered sibling marriage, influenced by the legend that the god Osiris married his sister, Isis, to maintain a pure bloodline. There were even instances of "double niece" marriages (defined as when a man marries a girl who was the offspring of his brother and sister).
King George III of England, whose reign was famously marked by losing the American Revolution, likely had a genetic disorder that affected his mind more noticeably than his body. He is believed to have suffered from porphyria, a disease that makes a patient’s urine bluish-purple and causes bouts of insanity (though arsenic poisoning and bipolar disorder have also been suggested as possible causes).
George III routinely checked out from his royal duties to escape to seclusion and private recovery at Kew Palace. He was prone to babbling delusions in his later life and subjected to extreme treatments including straight jackets, leeching, and ice baths to calm him. Modern medical testing shows porphyria was common in the highly inbred House of Hanover, to which King George III belonged.
George III spent the final decade of his reign in hiding and eventually lost his vision and hearing.