I am not sure how to translate
Augustus affirmāvit genūs suum ab Iove ortum esse.
One can logically conclude that this much of the sentence is correct...
Augustus affirmed that ... from Jupiter had arisen.
genūs can only be genitive singular, and suum can only be accusative.
If I try to put this together the only way to put this together is
Augustus affirmed that his knee had arisen from Jupiter.
My question is if this is actually the correct translation, then why didn't the author use genū instead of genūs?