Which two Game Boy games introduced Pokémon breeding to the series?

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

Released for the Game Boy Color in Japan in 1999 and internationally in 2000, Gold and Silver added the Johto region and 100 new Pokémon. They also expanded the original mechanics with breeding, held items, Pokémon eggs, a day-and-night cycle, and a real-time clock.

Breeding takes place at the Pokémon Day Care. Compatible Pokémon can produce an Egg, which hatches after the player travels a required distance in steps. The resulting Pokémon can inherit traits such as moves and, in some cases, species-related characteristics.

Pokémon Crystal, released after Gold and Silver, refined Generation II but did not introduce breeding. Ruby and Sapphire later expanded the system with additional abilities, moves, and breeding possibilities in the Hoenn region.