Which Pokémon game pair introduced breeding to the main series?

Pokémon Gold and Silver introduced breeding to the main Pokémon game series.

Released in Japan in 1999 and internationally in 2000, the games added the Pokémon Day Care system’s breeding mechanics. Players could leave compatible Pokémon at the Day Care and receive an Egg, which would hatch after the player walked a required distance.

Breeding expanded team building and introduced a way to obtain baby Pokémon and inherited characteristics. The system also made it possible to pass down certain moves and, in later games, additional traits and abilities.

Players sometimes associate breeding with later competitive Pokémon because those games greatly expanded the mechanics. However, the basic feature began in Generation II, alongside the introduction of Pokémon Eggs and baby Pokémon such as Pichu and Togepi.