One copy is inherited from each parent, neither of whom would normally have the disease. Autosomal dominant means that one copy of a defective gene is enough to cause disease. So, a person who inherits the defective gene from a parent will have the disease, as will the parent. Inheritable myopathies passed on in an autosomal dominant pattern can be easy to trace through the family tree. But in reality, one or both parents might be carriers who silently harbor a genetic mutation.
Many parents have no idea they're carriers of a disease until they have a child with the disease.
These are called spontaneous mutations, and after they occur, they can be passed on to the next generation. A good way to find out more about your risk of inheriting or passing on an inheritable myopathy is to talk to your MDA clinic physician or a genetic counselor.