John Ernest I, Duke of Saxe-Weimar
Johann Ernst I, Duke of Saxe- Weimar( 21 February 1594 in Altenburg – 6 December 1626 in Sankt Martin, Hungary), was a duke of Saxe- Weimar.
Johann II, Duke of Saxe-Weimar
Johann II, Duke of Saxe- Weimar(" Johann Maria Wilhelm")( 22 May 1570, in Weimar – 18 July 1605, in Weimar), was a Duke of Saxe- Weimar and Jena.
John Frederick, Duke of Saxe-Weimar
Duke John Frederick of Saxe- Weimar( 19 September 1600 in Altenburg – 17 October 1628 in Weimar) was a Duke of Saxe- Weimar.
William, Duke of Saxe-Weimar
Wilhelm, Duke of Saxe-Weimar (Altenburg, 11 April 1598 – Weimar, 17 May 1662), was a duke of Saxe-Weimar.
Wilhelm was the fifth (but third surviving) son of Johann, Duke of Saxe-Weimar, and Dorothea Maria of Anhalt.
He was brother to Bernard of Saxe-Weimar, a successful Protestant general in the Thirty Years' War, and to Ernest I, Duke of Saxe-Gotha (later Altenburg), a successful and well regarded ruler known as "The Pious".
John Ernest II, Duke of Saxe-Weimar
John Ernest II( 11 September 1627, in Weimar – 15 May 1683, in Weimar), was a duke of Saxe- Weimar.
He was the second but eldest surviving son of William, Duke of Saxe- Weimar, and Eleonore Dorothea of Anhalt- Dessau.
Dorothea Maria of Anhalt
Dorothea Maria of Anhalt (Dessau, 2 July 1574 – Weimar, 18 July 1617), was by birth a member of the House of Ascania and princess of Anhalt.
After her marriage, she became Duchess of Saxe-Weimar.
Dorothea Maria was the sixth daughter of Joachim Ernest, Prince of Anhalt, but second-born daughter by his second wife Eleonore, daughter of Christoph, Duke of Württemberg.
Ernest Augustus II, Duke of Saxe-Weimar-Eisenach
Ernst August II Konstantin, Duke of Saxe- Weimar- Eisenach( 2 June 1737 – 28 May 1758), was a duke of Saxe- Weimar- Eisenach.
Johann Ernst III, Duke of Saxe-Weimar
Johann Ernst III, Duke of Saxe-Weimar (Weimar, 22 June 1664 – Weimar, 10 May 1707), was a duke of Saxe-Weimar.
William Ernest, Duke of Saxe-Weimar
William Ernest, Duke of Saxe- Weimar( 19 October 1662 – 26 August 1728) was a duke of Saxe- Weimar.
Ernest Augustus I, Duke of Saxe-Weimar-Eisenach
Ernest Augustus I, Duke of Saxe- Weimar( German:" Ernst August I"; 19 April 1688 – 19 January 1748), was a duke of Saxe- Weimar and, from 1741, of Saxe- Weimar- Eisenach.

Question: Who is the maternal grandfather of William, Duke Of Saxe-Weimar?
Answer: Joachim Ernest, Prince of Anhalt