"In the absence of mutagens, an average gene will mutate about once in a million generations. ~10^16 cell divisions occur in a typical life span so somewhere in cells that are part of you each of your genes has mutated 10^10 times (hence cancer). Only mutations in the germ line can be passed on to the next generation. You have about 50 mutations that your parents dont have (mutations that happened in the egg or sperm that made you up)-each of your parents in turn have passed on ~50 mutants that their parents dont have."

Source: an email from my undergrad genetics professor