KrierScience

In an attempt to keep the royal blood pure, the royal family increased the occurrences of hemophilia which may account for the unexplained deaths of some royals throughout history. Hemophilia has been documented in the family pedigree of the royals starting with Queen Victoria. 

The BIOLOGY of it:  Hemophilia is a sex linked recessive disorder. Sex linked traits are carried  on the X chromosome.  For recessive disorders to show a person must inherit two copies of the recessive gene, one from the mother one from the father.  However in the case of disorders carried on the X chromosome, males only get one copy so if that copy carries the recessive disorder they will show the recessive disorder.