Which Pokémon games first introduced breeding in the core series?

Pokémon Gold and Silver first introduced breeding in the core Pokémon game series.

Released for the Game Boy Color in Japan in 1999, Gold and Silver added a Day Care system in which compatible Pokémon could produce an Egg. The Egg typically hatched into a young Pokémon related to the mother’s species, creating a new way to obtain and train Pokémon.

Breeding became a major part of later gameplay because Eggs could pass down certain moves, abilities, individual values, and other inherited characteristics. Ditto also made the system more flexible, although some Pokémon groups, including most Legendary Pokémon, cannot breed.

Players sometimes associate breeding with later games because those titles expanded the mechanics substantially. However, the feature’s core-series debut was in Generation II. Gold and Silver also introduced a real-time clock, held items, the Dark and Steel types, Shiny Pokémon, and 100 new species, making them a major expansion of the original formula.