Princess Sophia Dorothea of Prussia
Princess Sophia Dorothea of Prussia  (25 January 1719 – 13 November 1765) was the ninth child and fifth daughter of Frederick William I of Prussia and Sophia Dorothea of Hanover.
By marriage, she was a Margravine of Brandenburg-Schwedt.
Princess Caroline-Mathilde of Denmark
Princess Caroline- Mathilde of Denmark( 27 April 1912 – 12 December 1995) was a daughter of Prince Harald of Denmark and granddaughter of King Frederick VIII of Denmark.
As the wife of Knud, Hereditary Prince of Denmark, she became Hereditary Princess of Denmark.
Maria of Mecklenburg-Schwerin
Marie of Mecklenburg, born sometime between 1363 and 1367, but probably by 1365, dead after 13 May 1402, was a duchess of Pomerania.
She was the daughter of Duke Henry III, Duke of Mecklenburg( death 1383) and the Danish princess Ingeborg of Denmark, Duchess of Mecklenburg( death 1370), was Duchess of Pomerania.
Princess Friederike Luise of Prussia
Princess Friederike Luise of Prussia( 29 August 1714 – 4 February 1784) was a daughter of Frederick William I of Prussia and Sophia Dorothea of Hanover and Margravine of Brandenburg- Ansbach.
Anna of Brandenburg
Anna of Brandenburg (27 August 1487 – 3 May 1514) was a German noblewoman.
Margravine Anna was the daughter of John Cicero, Elector of Brandenburg and Margaret of Thuringia.
She was born in Berlin, Brandenburg, and died in Kiel, Holstein.
Dorothea of Brunswick-Lüneburg
Dorothea of Brunswick-Lüneburg (1 January 1570 – 15 August 1649) was a daughter of Duke William "the younger" of Brunswick-Lüneburg and his wife, Dorothea of Denmark, Duchess of Brunswick-Lüneburg.
She married Count Palatine Charles I of Zweibrücken-Birkenfeld.
They had four children:
Anna Amalia, Abbess of Quedlinburg
Princess Anna Amalia of Prussia (9 November 1723 – 30 March 1787) was Princess-Abbess of Quedlinburg.
She was one of ten surviving children of King Frederick William I of Prussia and Sophia Dorothea of Hanover.
Dorothea of Denmark, Duchess of Prussia
Dorothea of Denmark (1 August 1504 – 11 April 1547), was a Duchess of Prussia by marriage to Duke Albert, Duke of Prussia.
She was the daughter of King Frederick I of Denmark and Anna of Brandenburg.
Dorothea of Brandenburg, Duchess of Saxe-Lauenburg
Dorothea of Brandenburg( 1446 – March 1519) was a princess of Brandenburg by birth and by marriage Duchess of Saxe- Lauenburg.
Dorothea of Saxe-Lauenburg
Dorothea of Saxe-Lauenburg (9 July 1511 – 7 October 1571) was queen consort of Denmark and Norway by marriage to King Christian III of Denmark.
She was known to having wielded influence upon the affairs of state in Denmark.

Question: Who is the maternal grandfather of Dorothea Of Denmark, Duchess Of Prussia?
Answer: John Cicero, Elector of Brandenburg