How is egg move inheritance determined in Gen IV?

Question

Hi all, I'm having trouble with a breeding project and hoping for some help. Thanks in advance. I'm playing Pokemon Diamond and trying to breed a Misdreavus with the moves Nasty Plot and Ominous Wind inherited as egg moves from its father, a Spiritomb. His moveset was as follows, all learned from leveling up:

Hypnosis
Dream Eater
Ominous Wind
Nasty Plot

But I just bred a few Misdreavuses with the following moveset:

Growl (lv. 1 move)
Psywave (lv. 1 move)
Ominous Wind (egg move)
Dream Eater (egg move I guess, but I didn't even think this was possible?)

I wanted it to learn Nasty Plot as its second egg move, not Dream Eater. I made my Spiritomb forget Dream Eater so he couldn't pass it down, but now I've hatched a baby Misdreavus who only knows Growl, Psywave, and Ominous Wind. Nasty Plot isn't showing up at all, even though her dad knows it and it should work as an egg move. What am I doing wrong? How do I get her to be hatched knowing Ominous Wind and Nasty Plot? Thanks for any help!

Comments

https://pokemondb.net/pokedex/misdreavus/moves/4

There must be some reason why this site didn't mention Nasty Plot as an egg move for misdreavus, but later has mentioned the compatibile parents list...
Even this has kept me wondering...

Answer 1

In Gen 4, Nasty Plot is only an egg move for Misdreavus in HG/SS, which is why that isn't working in Diamond (source). Unfortunately, PokemonDB doesn't properly address these edge cases.

Dream Eater is being inherited because in Gen 4 (and all games prior to Gen 6), the baby will inherit any TM or HM moves known by the father which the baby is also compatible with. It only matters that the move is a TM or HM in the game in question, not that the Pokemon actually learned it using a TM or HM.

Comments

Thank you so much for this detailed response! I will try trading the parents over to my SS copy and breeding them there for the egg moves I want.