According to the article, 10 to 15% of  all colon cancers are caused
 by the gene.  To keep things simple, say 


      Pr(they have the gene | person has colon cancer) = .12


      Pr(person has the gene) =1/200


For those people with the gene the risk of getting colon cancer is
70-90%.  Estimate:


       Pr (getting cancer | person has the gene) = .8


So one can ask for Pr((getting colon cancer|does not have the gene)
(this seems to require a bit of algebra)..  Also one can derive
Pr(getting colon cancer).