Translation:Η πρόγονός μου.
This is always the case when the possessive follows a noun that is stressed in the antepenultimate syllable. Other examples:
το αυτοκίνητό μου
ο πρόεδρός μας
η παράστασή του
The second accent has been omitted many times by mistake in this course but the contributors are aware of the problem and will be fixing these sentences in the new tree.