Originally Posted by Andezzar
Is I said it is RAW but breaks my suspension of disbelief.
Wishing for something outside the list in the spell description can be a really bad idea, depending on he DM.[/edit]
Why are you confusing stats with fluff? The Half-Dragon template, given that dragons can BREED WITH ANYTHING, to me, is a retrovirus that improves ANY dna given to it. It is not a new set of DNA. At.. the only way it could be DNA would be if it had extremely sophisticated iterated decisionmaking trees built into it that modified DNA in certain 'draconic' ways without making an unviable creature - which is essentially describing a universal retrovirus in DNA form anyway.
It's like, say, a supersoldier serum. It turns Some Guy into Captain America, complete with wings and fire breath. And his children are also Captain America, although after a while they simply turn into Bucky (Draconic template) and a while after that they eventually turn into 'with training, could be Scarlet Witch' (Sorcerer).
It just only works on fetuses, instead of grown men (and with the right spells or prestige classes, you can apply it to grown men too).
Saying 'Half-Dragon' is simply a simplistic way of looking at a highly advanced biothaumaturgic improvement engine based on breeding - probably developed by the same sorcerers who turned themselves into dragons in the first place, as part of the method to insure that their antecedents' DNA never became degenerate due to their genetic tampering.
Half-Fiend is basically the same, since all Fiends are force-bred warrior constructs designed to aggressively reproduce by claiming souls. Now, you combine two retroviruses, you're asking to become a Gibbering Horror - but if both retroviruses are sufficiently advanced and have sufficient decisionmaking trees, they will simply mesh together and both provide their respective benefits (although DnD doesn't model reality super correctly here, as their potency should be reduced in combination as they would probably both modify the same thing in the same way and thus not be as effective as simply adding) without creating sudden horrible drawbacks.
And anyway, the fluff answers this question like 50 times. Half-Dragon doesn't just mean that one of your parents was a dragon, it says so in the draconomicon - it means that one of your parents was a dragon, one of your GRANDPARENTS was a dragon, or due to some weird fluke you got the full package instead of just being a silverbrow human or draconic human as your ancestors blood (read: magic and retrovirus pieces) combined oddly and gave you the half-dragon template. Half-Dragon is just the bigger, sexier, more magical oomph version of Draconic, and draconic doesn't even necessarily need a dragon in your bloodline at all.
Conflating fluff and mechanics is bad. It leads to limiting the things you can have in your game drastically. Very drastically. Don't do it.