What medical condition did Czar son have?

The only son of Czar Nicholas II of Russia was Alexei Nikolaevich, who was born on 12 August 1904. He suffered from haemophilia B, an inherited bleeding disorder in which the blood does not clot properly. This condition made him prone to internal bleeding and bruising, which could be life-threatening. Alexei's haemophilia was passed on to him by his mother, Empress Alexandra, who was a carrier of the disorder.