So I have not had a flu shot in 20 years, however I have not had the flu in 30 years. So I just wonder how genetics come into the equation?

I usually always get the flu shot in mid October, in 2019 in early October prior to getting the shot, I got very sick, didn't go to the doc for several days, wife finally made me go, tested positive for both that season's A and B strains at the same time. It was not fun. My temp was above 102 for almost 4 days, Tylenol had no effect.
I also had the flu in the spring of 2019 after having the flu shot the previous year as well. It was bad, but not as bad as getting 2 strains at the same time without the shot. Moral of the story, always get the flu shot and get it early. Yes, you can still get the flu with the shot, some years the vaccine it better than others and there are multiple strains. Get in regardless. I'm not sure what the odds of catching 3 flu strains in calendar year are, but I did it and don't want to do that ever again.