Prince William Victor Charles Augustus Henry Sigismund of Prussia, was the second son of Prince Henry of Prussia and Princess Irene of Hesse and by Rhine. He was the nephew of Kaiser Wilhelm II and Tsarina Alexandra of Russia. As the great-grandson of Queen Victoria through both his parents, he was the only one of three brothers who did not have the hemophilia common among her descendants.

Haemophilia figured prominently in the history of European royalty in the 19th and 20th centuries. Queen Victoria and her husband, Prince Albert, of the United Kingdom, through two of their five daughters – Princess Alice and Princess Beatrice – passed the mutation to various royal houses across the continent, including the royal families of Spain, Germany, and Russia. Victoria's youngest son, Prince Leopold, Duke of Albany, also had the disease, though none of her three elder sons did. Tests on the remains of the Romanov imperial family show that the specific form of haemophilia passed down by Queen Victoria was probably the relatively rare haemophilia B. The presence of haemophilia B within the European royal families was well-known, with the condition once popularly known as "the royal disease".