Europe has been controlled by royal families for hundreds of years, setting up strategic marriages to boost the number of their allies. But sometimes these unions resulted in inbreeding, which meant family illnesses were easily passed down the bloodline, as well as any deformities.
Children inherit a mixture of the combined genetic material of their parents. But when the gene pools in two people are too much alike, there's a much higher chance the child will inherit something sinister.
Queen Victoria and Prince Albert with five of their children. (Corbis via Getty Images)
Let's not forget Queen Elizabeth and Prince Phillip are third cousins, though it's not a fact the couple tend to publicize.
Thankfully, these days royals are allowed to marry "commoners". The rule change meant Prince William could marry Kate Middleton and Prince Harry could wed Meghan Markle, among other popular royal unions.
Let's take a look at the most famous royal inbreeding examples of all time.
Queen Victoria liked to match her children with the offspring of royals from other European countries, leading her to be called the 'Grandmother of Europe'.
Queen Victoria at her Diamond Jubilee. (The LIFE Picture Collection via Getty)
The Queen herself married Prince Albert of Saxe-Coburg-Gotha, who was her first cousin (he was the son of her mother's brother). The couple first met when they were both just 16 and their mutual uncle Leopold came up with the idea that they should marry.
When it comes to royal blood, the most devastating impact of inbreeding has been haemophilia, a disorder that prevents blood from clotting and can be fatal.
Though Queen Victoria had haemophilia, she didn't face any kind of suffering. But she managed to pass the disorder to her children and grandchildren. Prince Albert had no sign of the disease, so he was unlikely to have been the one who passed the gene on.
One of their children, Leopold, Duke of Albany, was a haemophiliac who died at 30 from blood loss after he slipped and fell. Five of her grandchildren also died due to complications of haemophilia.
Victoria's daughter Alice carried the gene and passed it to three of her children, one of whom died when he was only two years old. Alice's daughter Alix, Empress Alexandra Feodorovna of Russia, passed the gene to her son Alexei and his diagnosis changed the course of Russian history due to the involvement of Rasputin, who came in as a "healer" for Alexei.
Princess Alice (1843 - 1878), daughter of Queen Victoria, with her children. (Getty)
Alexei suffered from prolonged bleeding in his childhood but, of course, that wasn't how he died. He was tragically murdered at the age of 13 in 1918, along with his parents and four sisters, in the aftermath of the Russian Revolution.
King George III
George III was known as "Mad King George" due to mental illness (most experts believe he was suffering from bipolar disorder). He's also known for ruling England at the time when they lost their main colony, America.
He was an ancestor of Queen Victoria's and is also a product of inbreeding, which is believed to have caused his disorder. Queen Victoria was the only child of Edward, Duke of Kent, who was King George III's fourth son, making him her grandfather.
George III (1738 - 1820) reigned from 1760-1820. (Getty)
Back then, nobody really knew what was wrong with the King apart his signs of "madness." It was said the King sometimes could not stop talking and, other times, he'd have convulsions.
Recent studies have pointed to patterns in the King's handwriting which, during his "manic episodes", was different to how it appeared when he was "normal". Doctors would try to treat his affliction in all sorts of ineffectual ways, such as putting him in a straitjacket and ice baths.
He also suffered from skin conditions which were treated with medicine made from a substance called Gentian, which is believed to have been the reason why his urine turned blue. At the time, doctors believed the reason the King often had blue urine was tied in with his "madness."
Ferdinand of Austria: The Habsburg Jaw
Ferdinand I was the son of Emperor Franz II and Marie-Therese, who were first cousins. He had the disorder known as "hydrocephaly", which means he had a large skull due to the presence of water in the brain. He also possessed what was known as the "Habsburg jaw", which was common among his predecessors, as well as epilepsy.
Portrait of Emperor Ferdinand I (1503-1564). (Getty)
A typical "Habsburg face" included a large head, long nose, bulbous lower lips and jutting jaw, as was said to be a direct result of inbreeding. The Habsburgs, a German-Austrian ruling family, made strategic marriages to cement their power, often wedding close relatives. The offspring of these marriages were known for their peculiar features.
Researchers from Spain's University of Santiago de Compostela studied 15 members of the so-called Spanish Habsburgs. They found that Spain's Charles I, Charles II and Philip IV showed five of the seven tell-tale features of "mandibular prognathism" (protruding jaw).
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.
Portrait of the Emperor Charles V (1500-1558). (Heritage Images/Getty Images)
Charles I, who was also known as Holy Roman Emperor Charles V, was said to have "a long, cadaverous face and a lopsided mouth (which drops open when he is not on his guard)," wrote Italian diplomat Antonio di Beatis in 1517.
Charles II had the misfortune of being the child of his father and his father's niece. Not only were his parents closely related, but they came from a long line of very closely related couples. Geneticists believe that the inbreeding evident in Charles II – where an individual has two identical copies of the same gene due to related parents – is almost the same as for a child born of incest.
Portrait of Charles II of Spain, 1680-1683. (Heritage Images/Getty Images)
The story of the Spanish Habsburgs ended in 1700 because, while Charles II was married twice, neither marriage produced any children.
It's not likely the Habsburgs would have been too thrilled to learn that their inbreeding was referred to today as similar to a "human laboratory" – nor would they have entertained the notion that what they assumed was clever strategic marriages would result in illness and deformity.