Question

We are learning about nucleic acids, proteins, DNA & RNA, and the nitrogenous bases. I know that A & G are purines, and C,T, & U are pyrimidines. I know that within these bases is where genetic mutations occur and diseases and formed, so I was just wondering which group, the purines or pyrimidines, has a greater tendency to cause disease? And what type of diseases(auto-immune, nuerological, etc.) each group(or base) causes? We didn't learn about this in class, and its not on a test or anything, I'm just curious as to how the bases and families relate to certain dieseases. Thank you