This is from the MGFA site...(the link is above the main forum page where is says "useful site for myasthenia gravis"...

"Is MG hereditary?
Researchers are uncertain of the cause of MG, but do not believe that it is hereditary. A genetic predisposition to autoimmune diseases can run in families, but MG is not thought to be directly hereditary."

Although in Alberta, there is a curious case that I am aware of where both male siblings have MG. (The sister is keeping her fingers crossed she doesn't get it).

Lydia