The ratio of shiny to non-shiny IV spreads in Gen I and II is 1/8192 (it's technically incorrect to say this is the "chance" of finding a shiny due to how the RNG works, but it's close enough). However, due to a quirk in the Gen I RNG, normal wild encounters in grass, caves, or water will never have an IV spread that results in shininess, and so only stationary and fishing encounters, in-game trades, and gift Pokémon can have shiny IVs in Gen I.
