4.3 In Exercise 4.1, suppose that when the doctor had asked you about your medical history, you did not know that there had been a strong incidence of cancer in your family. On further investigation, suppose that you found that three out of four of your grandparents had died early of the same type of cancer for which you are being tested and that your father had been misdiagnosed when he died. In fact, your father actually died from this form of cancer. Now you have to question whether the figure of one in a million that applies to the population at large applies to you. It probably does not appIy to you. The chances of getting this type of cancer may be much higher for you than for others, because of something related to your genes. So the incidence of cancer for your subpopulation is not one in a million, but perhaps it is one in a thousand. Now what is your chance of having this type of cancer given your positive test result?