German Emperor (Kaiser), ruled January 18, 1871-1888 and King of Prussia, ruled 1861-1888
The election of Wilhelm 1., King of Prussia, as German Emperor (1871) was by vote of the Reichstag of the North German Confederation, on the initiative of all the reigning Princes of Germany. The Imperial dignity is hereditary in the House of Hohenzollern, and follows the law of primogeniture.
It may be possible to confirm family relationships with Wilhelm I by comparing test results with other carriers of his Y-chromosome or his mother's mitochondrial DNA.
Y-chromosome DNA test-takers in his direct paternal line on WikiTree: