It is instinct for any dog to bite....you need to teach to be gentle!
Mitzy was, and still does nip at you when playing, and she always used to go for our feet. I ended up buying us all slippers to protect our feet. She does it now, but she is gentle, and i know she means no harm. The way to teach them to be gentle is hand feeding them, and everytime they are gentle when taking from your hand, say 'gentle' then if they start rough playing, tell them gentle.
Molly, on the other hand, i think because she has always been with another dog, is so gentle, but she is such a growler...not sure what scares me most!
I am by no means an expert, but this helped me!