Genes are a factor but not as great as you like them to be. The I have bad Genes argument is a copout towards working towards a better life. Your environment, has a major effect as well, and you have luck too. Changing your environment helps your odds.
Lets say everytime you smoke a cigarette you have a 1 in 500,000 chance of getting lung cancer. Lets say your genes make you more resistant so you may have 1 in 600,000 chance instead. So if you have good genes and you smoke a packs of cigarettes a day that is 20 chances in that 1 in 600,000 a day. If you have the average genes, and you don't smoke then you may get 1 chance every week from second hand smoke.
The evidence of the person who lived a long life despite having a risk factors may be due to just dumb luck, combined with other positive lifestyle choices not mentioned to get the overall odds up.
Also what you may call a bad Gene isn't necessarily a bad gene, but they are designed for a life style that we are not living.
There are people who keep on eating junk food and stay skinny, their body has a high metabolism. With our culture that seems like a good thing, however for these people if they are late for their meal or cannot eat, they merely go into a panic, their body had used up more energy then they took in, and they just used that energy for silly things, such as shaking their leg while sitting, or creating more body heat. Then you have someone with a slower metabolism, that means they will be tend to be heavier, and store extra fat, this extra fat can cause health issues. However if they are unable to eat for a while they are not in such a shock. Now if you have a slow metabolism, and you need to manage the risks of being fat, then you need to exercise, to force your metabolism to go up for while and burn fat, and/or adjust your diet to insure you are just taking in the amount your body needs.
That Gym membership doesn't change your Genes, but if you use that Gym membership, it will help you work with what your genes had evolved you to do.