Original: Ernest I, Duke of Saxe-Gotha (1601-75) had married Elisabeth Sophie, the only child of John Philip, Duke of Altenburg and Gotha (1597-1638), the eldest son of Frederick William I/PERSON. When Elisabeth Sophie's cousin Frederick William III, Duke of Altenburg, died unmarried 1672, the entire first Altenburg line went extinct in male line, opening a succession strife.
Simplified: Ernest I, Duke of Saxe-Gotha (1601-75) had married Elisabeth Sophie, the only child of John Philip, Duke of Altenburg (1597-1638). Elisabeth Sophie's cousin Frederick William III, Duke of Altenburg, died unmarried 1672.