Pokémon breeding/Games/How to breed/Inheriting stats/Generation II

In Generation II games, the baby inherits its stats from the parent of the opposite gender, unless one parent is Ditto, and in that case it inherits the stats from Ditto. The Defense is passed, and also either the Special IV or the Special IV plus or minus 8 (plus for values in the 0-7 range; minus for values in the 8-15 range); the Special IV has a 50% chance of remaining the same. Attack and Speed IVs are determined entirely at random, whereas HP is determined by the IVs of the other four stats, using the same formula for wild Pokémon. The probability of a Pokémon having the same IVs as its opposite-gender parent is therefore 1/512.

If the Defense IVs of two partners are the same, and the Special IVs are either the same or differ by 8, the two Pokémon are incompatible. This is due to the high likelihood of a family relation, hence the inability to breed.