Well I've taken in very nice rats from others that turn completely protective of their babies on their first litter and would bite me for even trying to touch them.
All 15 of my adult females were handled as pets from pinky size and now that they are breeders will not bite me for any reason unless I hurt them.
My males are the same, when I open a tub they meander up and crawl into my palm hoping I'll hold them.
Genetics plays a huge role in how they react when they get babies, I swore the last weanling female I raised up was going to be a sweetheart, but as soon as she dropped babies she became protective.
This girl came from a friend, not my stock, she is now history, a snake must eat biters, it's the law here, lol.