In the European families, you had the two wings of the Hapsburgs: the Spanish and Austrian. The Spanish Hapsburgs intermarried...a lot. They eventually died out, with the last Spanish Hapsburg king dying in 1700 or so. Which led to the “War of Spanish Succession” when the French Bourbon king tried to place a relative on the Spanish throne. By the way, the last couple of Spanish kings were drooling idiots.
The Austrian Hapsburgs fared somewhat better, but not all that great. They very much frowned on marriage outside of their tribal clique, but to my knowledge kept the 1st and 2nd cousins apart.
Since Queen Victoria had a gob of kids and married them off to everybody but the Hapsburgs, and then those kids all had their kids marry each other, it’s no surprise the Battenburgs (now Windsors) had genetic problems. Czar Nicholas II married his 1st cousin, Alexandra of Hesse, a union that resulted in a hemophiliac male heir. Nicholas II was 1st cousin to King George V, grandfather of Elizabeth II. When they dug up the remains of Nicholas’ family, they sought as many close family members as they could for samples to confirm identity through DNA. Of course, they hit up the Windsors. You’d think they’d use Queen Elizabeth, since she was a direct descendant of a 1st cousin. Nope; her husband, Prince Phillip was used since he was considered a closer match.
Land and money is the only difference between Prince Charles and your stereotypical inhabitant of Appalachia.
The additional birth-defect risk is said to be about that for a 40-year-old mother...about 2%.
With the knowledge we have today we could AVOID them and marry only our first cousins and sisters who do not have them in their DNA.
The great danger, then, would be "strangers" who have them but we don't know about them!
But my comment was about the Bourbonnaise, mostly, who tried to marry AWAY from the mainline ~ THEY STARTED OUT with a couple of dwarves and some idiots.
Later generations were marrying Magyars! Somebody figured out the rules for them.
I do know of a case where a French Bourbon descendant showed up in Sweden and the first thing the king did was give him one of his illegitimate daughters as a wife. The king himself had a Great Grandmother who was also a Bourbon descendant. It was 4 generations of separation, within the family, but the Swedes had this thing about just grabbing women from the village, so no doubt there was a lot of genetic variation in this pairing.
Some of these people were pretty earthy and knew intimately about how you raised pigs, and sometimes you have to breed out or you get thin pigs.
Nicholas and Alix were 2nd cousins (N's grandmother and A's grandfather were siblings), not 1st. Prince Philip was used because he's a descendant of Queen Victoria in the female line, as was Alix, and mitochondrial DNA inherits only down the female line. (Queen Elizabeth is of course descended down a male line from Queen Victoria.)
Hemophilia is a sex-linked recessive, so it's not related to inbreeding. Alix could have married an Australian aborigine and still would have had a 50% chance of having a hemophiliac son.
Charles II of Spain was indeed a mess due to inbreeding.
They used the dna of Prince Philip because they were looking for a gene that was passed through the mother. Prince Philip’s mother was a great granddaughter of Queen Victoria.
Neither Queen Elizabeth’s father nor her grandfather married relatives. The most recent close relatives to marry in the British royal family were first cousins Victoria and Albert circa 1840.