Nativity

Mutations are native when your body acknowledges it as the form you were born with. People born since P day are all native of their various mutations they have at the time of birth, though there are items out there that can alter your native form.
 
Any bodypart with a native form that is not human is resistant to change. Any time a change should affect that part, there's a 50% chance it will instead change to its native form(or not at all, if already native).
 
There are items that can adjust this resistance.
 
Children begin with powers based on their original native form. These CANNOT be unmastered or changed by ANY means, even revising the native form.