Despite the games having been out for a while now in Japan, a new discovery has been made just recently in Pokémon Black/White 2—one that changes how the games’ breeding system works, according to Serebii.net.
Since Pokémon Ruby/Sapphire, Pokémon have had “Abilities,” which are special attributes that have different effects during battle. The could include preventing the opponent from fleeing, or a natural protection against Critical Hits, or eliminating weather effects and so on. Abilities are passive in nature and activate automatically during battles.
Each Pokémon has a single pre-designated ability, and there’s no way of changing what this is. However, some Pokémon are capable of having one of two pre-designated abilities, and in this case, the ability is chosen at random when you encounter the Pokémon in the wild.
Up until now, the same also applied to breeding. If you bred two Pokémon and hatched a baby Pokémon that could learn two abilities, it had a 50-50 chance of being born with either of the two abilities.
In Pokémon Black/White 2, this has changed. During breeding, the baby Pokémon’s ability is now dependent on its mother. If the mother is a dual-ability type, the baby has an 80% chance of inheriting the ability its mother has, and a 20% chance of inheriting the other ability that the mother doesn’t have instead.
Additionally, you can now 100% guarantee that your Pokémon will inherit a nature from one of its parents as well. This is done by giving one of the parents an Everstone to hold. The parent that’s holding the Everstone will pass its nature down to the baby.
More details about breeding mechanics can be found at the Serebii.net link above.
Update: Nature inheritance details added. Thanks to Jazlynn Lim for pointing them out.