Let’s suppose I breed 2 Grookeys. Both have grassy surge and adamant natures. What will be the result of the child?
The chance of the HA passing is 60% if the mother (or non-Ditto parent) has its HA. If one of the parents hold an Everstone, its nature will always be passed to the child, otherwise, the child's nature is completely random.
Without Everstone: 60% * 4% = 2.4% or roughly 1/42 eggs With Everstone: Flat 60% chance