What new Pokémon mechanic did Pokémon Gold and Silver introduce for producing Eggs?

Pokémon Gold and Silver introduced Pokémon breeding as the mechanic for producing Eggs.

In these games, compatible Pokémon left at the Pokémon Day Care could produce an Egg. The Egg generally hatched into the species of the mother, while the father could pass down certain moves. Ditto provided a special breeding option because it can breed with most Pokémon outside the No Eggs Discovered group.

Breeding expanded the series beyond catching wild Pokémon. It allowed players to raise new Pokémon, pursue particular moves, and later work toward desirable abilities, natures, and statistics. The mechanic also connected naturally with the games’ new Day Care and Egg-hatching systems.

Gold and Silver added many other features, including a real-time clock, Pokémon that could hold items, berries, a day-and-night cycle, and the split of the Special stat into Special Attack and Special Defense. Breeding therefore arrived as part of a much broader second-generation expansion.