Which generation introduced Pokémon breeding in the core games?

Generation II introduced Pokémon breeding in the core games.

The mechanic debuted in Pokémon Gold and Silver, released in Japan for the Game Boy Color in 1999. Players could leave compatible Pokémon at a Pokémon Day Care and later receive an Egg, which would hatch into a young Pokémon.

Breeding added a new way to obtain species, inherited moves, and special characteristics. In the usual case, the Egg’s species follows the mother, while the father can pass down certain moves. Ditto provides an important exception because it can breed with most compatible Pokémon.

Breeding is not available for every species. Legendary and Mythical Pokémon, along with Pokémon in the No Eggs Discovered group, cannot normally produce Eggs. Generation II also introduced Baby Pokémon and Shiny Pokémon, making Gold and Silver a major expansion of the series’ collection systems.