Actually MS is not dominant hereditary. What scientists think is that some people inherit a genetic predisposition to having MS (though a group of genes in concert) but that it takes something to "flip the switch" and cause you to have MS (like a virus, environmental exposure, etc) . This is all still supposition because noone has been able to definitively identify the genes or prove the switch theory. It's the best they've got.

My mom has MS, I have it, my brother has it, and my sister may have it as well. We hit the jackpot in my family!