Have I missed something? I didn't notice the words "DNA" or "genetic" anywhere in the game. Vader talks about "memory flashes", I always kinda assumed he had some kind of process by which he extracted memories from Starkiller's dead brain and inserted them into a new living brain.
Also, the distant thunder body does wear the clothes Starkiller got stabbed in the back by Vader in. I still think Starkiller's been dead a long time and we've been playing a clone since halfway through TFU1.