The royal disease is hemophilia.
Hemophilia is called Royal disease because the hemophilia gene was passed from Queen Victoria a hemophilia B carrier who became the Queen of England in 1837 to the ruling families of Russia, Germany and Spain.
It's thought that Queen Victoria's gene for the hemophilia was likely caused by spontaneous mutation.
The royal disease was likely caused by a point mutation in F9, a gene on the X chromosome that encodes blood coagulation factor IX.
(A) Partial pedigree of the royal family, showing transmission of the mutation from Queen Victoria to Empress Alexandra and from Alexandra to Prince Alexei, her hemophilic son.
Hemophilia is a rare disorder in which the blood doesn't clot in the typical way because it doesn't have enough blood-clotting proteins (clotting factors).
When you have hemophilia, you might bleed for a longer time after an injury than you would if your blood clotted properly.
The best way to treat hemophilia is to replace the missing blood clotting factor so that the blood can clot properly.
This is typically done by injecting treatment products, called clotting factor concentrates, into a person's vein.