ok..
First off, there is more then one way to go bald. One way is to have a defective gene. its a recessive gene that is carried on the X chromosome.
What does that mean? Your MOM would have given it to you.
So your mom get some X from her mom and the other X from her dad, your grandpa.
So if your mom's dad is one of those types that is completely bald, not just receeding, but BALD.. then chances are near 100% that she carries that trait on her X chromosome. She isn't bald, and her hair isn't thinning because she likely got a good X from her mom... since the trait is recessive, she is fine.
You are not though... she passed alone one of those Xs to you. So you have a 50/50 change of having the same x chromosome that your grandfather did... so a 50/50 change of being bald like that too.
There are other types of hair loss in men though.. so just cuz you made it through that scare doesn't mean your hair wont creep back at the crown starting about age 25 and continue.