When duke of Saxe-Wittenberg died without an heir in 1422 , the Emperor Sigismund gave the duchy to Frederick IV of the House of Wettin . Frederick was already Margrave of MeiÃ en and Landgrave of Thuringia , and then became Frederick I , Elector of Saxony . When the last duke of Saxe-Wittenberg died without heir in 1422 , the Emperor Sigismund gave the duchy to Frederick IV of the house of Wettin , Margrave of Meissen and Landgrave of Thuringia , who thereby became Frederick I , Elector of Saxony .