Egg Group
Egg groups are categories which determine which Pokémon are able to interbreed. Similarly to the elemental types, a Pokémon may belong to either one or two egg groups.
Egg groups are decided by biological traits, which overall serves to allow Pokémon which are biologically similar to breed. However, there are some oddities which have been noted within egg groups: for example, the Ground egg group's standards are particularly broad, and many seemingly illogical pairings can be noted within it, such as Skitty and Wailord.
Egg groups are preserved along evolutionary lines; Ralts, Kirlia, Gardevoir, and Gallade are all in the Indeterminate group, while Pikachu and Raichu are in the Ground and Fairy. Baby Pokémon are the only Pokémon besides Shedinja, Nidorina, and Nidoqueen that are not in the same group as their evolutionary relatives; Nidorina and Nidoqueen are both in the No eggs Group, while Shedinja is in the Mineral Group, despite having no gender, and, as such, requiring Ditto to breed.
Breeding chains
Many Pokémon are included in two different egg groups and can form what are known as breeding chains. These link many unrelated Pokémon together, allowing moves that would not normally be accessible to certain Pokémon to be legal, like Heal Bell on Blissey.
TMs are also passed down through breeding from the father. Any move in the father's current moveset that can be learned via TM, egg move, or move tutor (in Crystal) will be passed down to the Pokémon hatching from the egg.
Egg groups
This is a list of egg groups by their index number in the games:
- Monster Group: Pokémon in this group are dinosaurian/kaijū-like in appearance and nature.
- Water 1 Group: Pokémon in this group are amphibious in nature.
- Bug Group: Pokémon in this group are insectoid (bug-like) in appearance.
- Flying Group: Pokémon in this group are avian (bird-like) in appearance.
- Ground Group: The largest group, Pokémon here are terrestrial in nature.
- Fairy Group: Pokémon in this group are petite and considered very cute.
- Plant Group: Pokémon in this group are plant-like in appearance.
- Humanshape Group: Pokémon in this group are fully bipedal.
- Water 3 Group: Pokémon in this group resemble aquatic invertebrates.
- Mineral Group: Pokémon in this group are crystalline or silicon-based.
- Indeterminate Group: Pokémon in this group are amorphous, having no definite form.
- Water 2 Group: Pokémon in this group are piscine (fish-like) in appearance.
- Ditto Group: Pokémon in this group are capable of breeding with all others (regardless of gender) aside from the no eggs group.
- Dragon Group: Pokémon in this group are draconic in appearance.
- No eggs Group: Pokémon in this group are unable to breed with anything.
