Ziggby,
It sounds like one of your grandparents might have had a dominant form of MD. (Weather it showed or not). Any idea what kind.? I am no expert but I've read countless pages on dominant and recessive genes. (I have Limb Girdle MD). In a classic dominant case only one parent has the "bad" gene. Kids have a 50-50 chance of inheriting the gene. One son could be healthy, another son affected, a daughter healthy and another daugter a carrier (she won't have symptoms but may give it to her kids 50-50). Sound familiar?
Let me tell you this . . . My dad had MD. His brother is heathy and so are his two grown kids. My dad's sister is healthy with 3 heathy kids in their forties).
Read up on dominant and recessive genes. Check out www.***usa.org.
Best of Luck