I think he has it because since his father, Mezu Uchiha and his mother Leshna Hyuuga gave birth to him, the chances are that he had Byakugan and that he can switch between his past Sharingan and Byakugan because he should have the blood line of Leshna's KG.
Plus, something would have to divide Madara from the rest of the Uchiha, and the Byakugan that he probably had would do so.
Thoughts?
Plus, something would have to divide Madara from the rest of the Uchiha, and the Byakugan that he probably had would do so.
Thoughts?