Board Thread:General Discussion/@comment-26580042-20160826010854/@comment-25926288-20160829171050

I believe that it's that you have to be born to wield your personal magic. Cora, Zelena, and Regina are all connected, so it's no surprise that they all have magic. Walsh specifically says that Regina was born with the gift of magic, like Zelena.

So, season 3b and onwards firmly declares that you have to be born with magic to wield your personal magic. (Yes, Amara had magic as her sons also had magic, at least Cyrus post-Genie did magic although that was primarily a crafted spell).