“I just discovered thru DNA testing that my “Dad” isn’t my “Father”. My Dad signed my birth certificate and raised me, but I’m not sure if he did it knowing I wasn’t his. My Father is dead, and my Dad never adopted me. So, what is my last name? Is it the same as my Dad because he signed the birth certificate, or is it the same as my Father because I wasn’t adopted? I was born in MA in 1960.”
Your last name is whatever is on your birth certificate, however if you wish to change it you should be able to easily do so through either your motor vehicle department, or your local courts, depending on in what state you currently reside.