The easiest argument against this is that dragons reproduce asexually. There isn't any way for them to get or give genetic material to anything else. What can be passed on (sort of) is their aether. But we've only seen that happen when the original Heavensward ate Ratatoskr's eyes and all their decedents have latent dragon aether. The other time is that according to Hrasvelger, he ate Shiva and the somehow caused her aether to stay with his. But none of that goes so far as to change people's appearances to become more draconic as a genetic trait.
