La bible aka bulbapedia à écrit
In Generation II, shininess is determined by the IVs of a Pokémon. If a Pokémon´s Speed, Defense, and Special IVs are 10, and its Attack IV is 2, 3, 6, 7, 10, 11, 14 or 15, it will be shiny. This determination allowed for compatibility with Generation I, as if a shiny Pokémon is traded back, then traded forward, it would retain its shininess. Despite the compatibility allowing for shiny Pokémon to learn Generation I TMs, provided it was a Generation I Pokémon, shiny Pokémon are slightly above average in terms of IVs.
Due to the probability of the IVs lining up in any of these manners to produce a shiny Pokémon, the chance of obtaining one in Generation II is approximately 1/8192.