How likely am I to get colon cancer if my father had it terminally?

I am very sorry to hear that your father passed due to colon cancer, especially at such an early age. To the average person in the United States, the lifetime risk of developing colon cancer is around 7%. The risk dose increase with age, and typically it forms from the lining inside the colon. Typically thought to start out as a small polyp (particularly adenomatous polyp), or overgrowth of the lining that grows out of control and eventually becomes a cancer. If left uncontrolled, it can spread to lymph nodes, and other organs of your body. Typically it can be caught early through regular screening such as colonoscopies. If detected early enough, the polyps can be removed before they de-differentiate into a cancer. What worries me about the history you gave me is that your father developed colon cancer at such a young age, and that it was so severe. There are some hereditary factors (inherited) that affect colon cancer. For instance if the individual has FAP (Familial Adenomatous Polyposis), there is a near 100% chance that they will develop colon cancer at an early age (around 40) if left untreated. Other hereditary factors include HNPCC (Hereditary Nonpolyposis Colorectal Cancer), and Gardners syndrome. Even without genetic evidence that you have one of these disorders, the fact that you had a family member with colon cancer before the age of 55 puts you at higher risk of developing it. I would definitely highly recommend making an appointment to see a gastroenterologist (GI doctor) to get plugged into the system for regular surveillance, so that a cancer is detected early if it develops. I wish you all the best.