Which pair of Pokémon games introduced the Johto region and Pokémon breeding?

Pokémon Gold and Silver introduced the Johto region and Pokémon breeding.

Released in Japan in 1999 for the Game Boy Color, Gold and Silver were the first main-series sequels to the original Pokémon games. Their story begins in Johto, a region located near Kanto and inspired by Japan's Kansai area.

The games added 100 new Pokémon, bringing the Pokédex total from 151 to 251. They also introduced breeding at the Pokémon Day Care, where compatible Pokémon could produce Eggs and pass certain characteristics to their offspring.

Gold and Silver added other lasting features, including Pokémon holding items, a day-and-night cycle, a real-time clock, genders for most Pokémon, and the Steel and Dark types. The original games introduced trading and battling, but breeding first appeared in this second-generation pair.