In Pokémon Gold and Silver, what feature first allowed Pokémon to produce Eggs?

Pokémon Gold and Silver first introduced Pokémon breeding, allowing compatible Pokémon to produce Eggs.

The feature was one of the major additions to the second-generation games, released in Japan in 1999 for the Game Boy Color. Players could leave compatible Pokémon at the Pokémon Day Care, where an Egg might eventually appear. Walking a set distance caused the Egg to hatch into a young Pokémon.

Usually, the hatchling belonged to the mother’s species, while the father could pass down certain moves. Ditto was an important exception because it could breed with most Pokémon outside the No Eggs Discovered group. Legendary and Mythical Pokémon generally could not breed.

Breeding also became important for collecting, competitive training, and obtaining baby Pokémon. It should not be confused with evolution: an Egg produces a new Pokémon, whereas evolution changes an existing Pokémon into another species. Gold and Silver also added held items, a day-and-night clock, Pokémon abilities’ predecessor mechanics, and the Dark and Steel types.