"His grandmother wears a blue skirt."
Translation:Anneannesi mavi bir etek giyer.
It doesn't have to do with the subject's gender, but actually which side of the family the grandmother is on....
Duo's maternal grandmother:
anneannesi = his
Duo's paternal grandmother:
babaannesi = his
And because this sentence doesn't give any clues or context about which side of the family, you can use either anneannesi or babaannesi :-)