It is said that the Dragonborn of Sancstoryl were created when Tiamat and Bahamut spilled and mixed their blood when they fought during the early days if the world, with it magically taking on new, humanoid forms. So, even though is is said that chromatic Dragonborn are said to have more of Tiamat's blood and metallic Dragonborn more blood of Bahamut, they don't particularly gravitate towards any alignment.
For Gem-Dragonborn meanwhile, it is said that they were first created by Sardior the Ruby Dragon on a whim, or at least what remained of his spiritual essence, after seeing the Dragonborn thriving in the coming centuries. Then, he decided to add some of his own essence to the mix, which resulted in Gem-Dragonborn being born.
The Dragonborn of Sancstoryl originally didn't have tails, but in the past two-thousand years, tails have become more common, and are even starting to outnumber the tailless in some areas. Tailless dragonborn nobility has even started to wear fake tails, due to changes in the beauty standard. Nonetheless, whether through sorcerous bloodlines, a connection to Illiadian Dorro, or some other reasons, it's actually unknown why tailed Dragonborn are becoming more common.
Dragonborn are most common on It'ada.