how to test the genetic contribution of modifier gene to the disease in multiple - (Jun/02/2007 )

HI guys,
I am doing a study that explores the genetic contribution of the modifier gene(name for X gene) to the disease age of onset and disease procession in multiple-pedigree from one founder. I want to detect how the SNP of the modifier gene affect the disease that has a major gene (A) identified already. i am kind of doubt which test is suitable here if i wanna to do association study to evaluate the B gene SNP and the disease status or disease age of onset? the other detailed information is that the major gene A has various repeat expansion leading to disease and the expansion size cause diffirent disease severity level.
It is kind of complicated. what i wanna to do is have a conclusion whether the modifier X gene SNP contribute the disease severity , or age of set when controlling for the A expansion size.
Any suggestion will be highly appreciated.
Thanks guys!

You mean in genetics part of this forum?
Would you please expalin it clearly? I am so confused.