Draconic sorcerers have dragon heritage mixed with some non-dragon ancestor.  So is a red dragon got busy with a blue Dragonborn, that’s a way to arrive at the situation you describe.
I am curious as to why we dont see "mixed race" dragons in the game. It could make for some very powerful encounters.
per my question on Chromatic and metallic dragons, A lawful evil Red Dragon pairing with a lawful Good silver dragon. Would they get both elemental resistances and breath weapons?
its silly question like this that keep me passing the time for the game, Like, Why did Shar send a lv one Cleric to steal a artifact from a bunch of psychopathic  murdering space pirates? Did she (Shar) run out of high lv Clerics or hell she could of sub contracted a high lv wizard to at least be able to teleport. Or even yet why didn't shar a God just get the artifact herself?
I cant tell you how many times I've seen Shadowheart one shoted from a intellect devourer in the first minute of landing upon Torel, and Im to believe she is the one Shar picked. lol
 I know I know step away dude step away, don't look behind the Curtin.