Prince William of Wales will marry his girlfriend Kate Middleton in Westminster Abbey on Friday, April 29, as you may have heard on every single television news channel in existence. In two days, then, Middleton will become the first "commoner" to marry a future English king in 450 years, since Anne Hyde married James I in 1660. (When Prince Edward married Wallis Simpson in 1937, he had abdicated the throne six months earlier.) Some of the discussion around the wedding, then, has focused on how Kate Middleton will bring "new genes" into a family that has long intermarried with other royal European families. After all, many people have heard of how intermarriage led to hemophilia in Queen Victoria's descendants. (Hemophilia has sometimes even been referred to as "the royal disease.") The dangers of royal intermarriage go beyond hemophilia, though, and have even extinguished some of Europe's most famous dynasties.
Two years ago, scientists from the Universidad de Santiago de Compostela in Spain published the results of a genetic analysis of the Spanish Habsburg dynasty, under whom Spain reached its peak of world power in the 16th century. Intermarriage in the Spanish Habsburgs was quite common, even by the standards of the day, including "two uncle-niece marriages...one double first cousin marriage...one first cousin marriage...two first cousins once removed marriages, one second cousin marriage and two third cousin marriages." (And the dynasty only lasted two centuries!) The authors found that the constant inbreeding led to a "remarkable" reduction in the survival of Habsburg children until age 10: for example, in the Habsburg dynasty, children of a first cousin marriage were on average 18% less likely to survive to age 10; in modern first cousin marriages, the number is less than 5%. Furthermore, they found that the genes of the last Habsburg king, Charles II, were actually slightly more intermarried than a child of a brother-sister relationship, leading to two different debilitating genetic disorders. One of his many debilitations was impotency, which ended the dynasty. Small wonder, then, that he was nicknamed "Charles the Hexed."