What Breed Is My Dog?

This is Dot, she showed up on our porch one day looking lost and acted like she had been beaten. She has been a part of our family for over a year now and is very smart. We know she is most likely part white Lab, but she is completely white with the exception of her spot on her nose. She has bright blue eyes. And she also looks like her tail was either taken partly off or she was born that way. What other breed may she be?