Normal Ability - The mechanics of which Ability is acquired upon hatching a Pokémon Egg (in instances when a hatched Pokémon may have one of two Abilities) varies from game to game. In all games up to and including Pokémon HeartGold and SoulSilver, a Pokémon hatched from an Egg has a 50% chance of acquiring either Ability.
However, starting from Pokémon Black and White, there is an 80% chance of the resulting offspring having the same "slotted" Ability as its mother and hence a 20% chance of the other Ability being passed down as long as the other parent is not Ditto.*
*Hidden Ability - If a female Pokémon has a Hidden Ability and is bred, it has a 60% chance of giving that Ability to each of its children. Each of the other Abilities has a 20% chance of being passed down. In Generation V, Hidden Abilities cannot be passed down from male or genderless Pokémon, or when breeding with Ditto. In Generation VI, this was changed so that male and genderless Pokémon have the chance to pass down their Hidden Ability only when bred with Ditto.*
*Normal Ability - 80% mother/female and 20% father/male.
Hidden Ability - Only female can downpass and 60%.*
Source - Bulbapedia
Hope this helps!