So before you look at the beautiful pic I have prepared, a few notes:
You know that I have that headcanon about Eleanora actually being a Divine Dragon? It's here because it makes thinking about this more confusing interesting.
Eliwood/Ninian; Roy/Sophia to maximise Dragon-ness.
Humans and Dragons are the same species; which is different from actual real life humans. Why? They can canonically produce fertile offspring. In reality, this is only possible if they are the same species due to karyotypes being very strange if different species interbreed. Also, we're assuming having a Dragon form is controlled by one (dominant, if Ninian is a good indication) gene. So the genes we're talking about might not be expressed. So yeah.
Elibe only, guys. Trying to factor in other verses is a viable cause of hair-ripping for poor Magyars.
Now you have read all that (It wasn't much tbh, Magyars would probably ramble on for an hour) you can look at our lovely work with a single gene about Feathers. Go question our sanity.
[spoiler=Here is another pic that calls my (and Magyars') sanity to question.]
I hope you had fun in my lecture.