User blog comment:Water Dragon Slayer Shawn/Theory on why Gajeel and Natsu couldn't pass Freed's barrier./@comment-7418318-20140625112926

I don't think they got several organs from the Dragons or could make exact copies, since a Dragon's organs would be much bigger and a different shape, but I too think that the Dragons infused or implanted something of their own into their "children's" bodies.

Considering that only a true Dragon can teach this magic, it makes the most sense to me. If it was just teaching, there would be at least a few DS around who learned the Magic from another DS and not a Dragon. When it was revealed that Laxus is an artificial Dragon Slayer who got his powers from being implanted with a lacrima, I became even more sure of this theory. And that Acnologia supposedly became more dragon-like from "bathing in Dragon blood" (though I don't take this literally) futher supports it. Zeref might have been needed to turn him into a full Dragon, but the silhouettes we got of Acnologia before transforming were already showing horns. In light of Wendy's wings those horns might just have been part of his Dragon Force, but I think there's more to it, but I digress.

Back on topic: Now since this tissue or whatever it is that was put into the DS's bodies was originally from the Dragon Parents, who are both quite old (unless timetravel or stasis are involved, Igneel is confirmed to be more than 400 years old) they would likely trigger the runes.