Frederick III, Duke of Saxe-Gotha-Altenburg
Frederick III, Duke of Saxe- Gotha-Altenburg( 14 April 1699 – 10 March 1772), was a duke of Saxe- Gotha- Altenburg.
Princess Luise Dorothea of Saxe-Meiningen
Luise Dorothea of Saxe- Meiningen( 10 August 1710 – 22 October 1767) was a member of German royalty.
She was born in Meiningen, the daughter of Ernst Ludwig I, Duke of Saxe- Meiningen and Dorothea Marie of Saxe- Gotha.
She was the wife of Frederick III, Duke of Saxe- Gotha- Altenburg.
Ernest I, Duke of Saxe-Gotha
Ernest I, called "Ernest the Pious" (25 December 1601 – 26 March 1675), was a duke of Saxe-Gotha and Saxe-Altenburg.
The duchies were later merged into Saxe-Gotha-Altenburg.
He was the ninth but sixth surviving son of Johann II, Duke of Saxe-Weimar, and Dorothea Maria of Anhalt.
His mother was a granddaughter of Christoph, Duke of Württemberg, and great-granddaughter of Ulrich, Duke of Württemberg.
Dorothea Maria of Saxe-Gotha-Altenburg
Dorothea Maria of Saxe- Gotha-Altenburg( 12 February 1654 in Gotha – 17 June 1682 in Gotha), was a German princess member of the House of Wettin in the Ernestine branch of Saxe- Gotha- Altenburg.
She was the twelfth child and fourth daughter of Ernest I, Duke of Saxe- Gotha by his wife Elisabeth Sophie, the only daughter of John Philip, Duke of Saxe- Altenburg.
Frederick IV, Duke of Saxe-Gotha-Altenburg
Frederick IV, Duke of Saxe-Gotha-Altenburg (Gotha, 28 November 1774 – Gotha, 11 February 1825), was the last duke of Saxe-Gotha-Altenburg.
He was the third but second surviving son of Ernst II, Duke of Saxe-Gotha-Altenburg and Charlotte de Saxe-Meiningen.
After the death of his older brother August without sons (1822), Frederick (the only surviving male of the house) inherited the duchy of Saxe-Gotha-Altenburg.
Frederick fought - after military training - in the Napoleonic campaigns and was heavily wounded.
As a consequence of these injuries, he was constantly ill until his death.
Because of his illness, he traveled for a long time seeking a cure.
During these stays outside of his duchy, he left the government in hands of his secret advisor Bernhard August von Lindenau.
He only reigned three years and died unmarried; with him, the line of Saxe-Gotha-Altenburg ended.
After his death, his lands were repartitioned among his Wettin relations.
Ernst I of Saxe-Coburg-Saalfeld received Gotha, and changed his title to "Duke of Saxe-Coburg and Gotha", although the two duchies remained technically separate in a personal union.
Altenburg was thereafter ruled by the Duke of Saxe-Hildburghausen, whose dukedom was transferred to Saxe-Meiningen along with Saxe-Saalfeld, which Saxe-Coburg gave up in return for receiving Saxe-Gotha.
Princess Charlotte of Saxe-Meiningen
Princess Charlotte of Saxe- Meiningen( 11 September 1751, Frankfurt am Main, Free Imperial City of Frankfurt, Holy Roman Empire – 25 April 1827, Genoa, Kingdom of Sardinia) was a member of the House of Saxe- Meiningen and a Princess of Saxe- Meiningen by birth and a member of the House of Saxe- Gotha- Altenburg and Duchess consort of Saxe- Gotha- Altenburg through her marriage to Ernest II, Duke of Saxe- Gotha- Altenburg.
Ernest II, Duke of Saxe-Gotha-Altenburg
Ernest II, Duke of Saxe-Gotha-Altenburg (Gotha, 30 January 1745 – Gotha, 20 April 1804) was the reigning Duke of Saxe-Gotha-Altenburg from 1772 to 1804.
He was the third but second surviving son of Frederick III, Duke of Saxe-Gotha-Altenburg and Luise Dorothea of Saxe-Meiningen.
The death of his older brother Frederick in 1756 made him the heir to the duchy of Saxe-Gotha-Altenburg.
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".
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.
William Ernest, Duke of Saxe-Weimar
William Ernest, Duke of Saxe- Weimar( 19 October 1662 – 26 August 1728) was a duke of Saxe- Weimar.

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