Subject: Referring to medical molecular genetics, what determines the severity of a

I just received an A in my genetics class, but I just realized that I don't 
know why a particular disease/syndrome/condition is more severe in some cases 
than in others.  In other words, how does someone get a less severe case of 
multiple sclerosis than someone with an extreme case?  Does it have to do with 
the amount of expression of particular genes or proteins?  If so, what 
determines this amount or level?  Thank you so much! 

Re: Referring to medical molecular genetics, what determines the severity of a