I haven't read the book but I plan to but I definitely have an opinion about this.
I think genetics have determined my fate somewhat. My belief has always been that my genetics have determined me to be overweight but my actions have determined how overweight I got. It has been something I've wrangled with because I don't remember an abnormal child hood of eating anything out of the ordinary but I was 300 lbs when I was 14. I do remember eating an excess of "normal" things though.
Ever since I was 4, I was "chubby" but beyond being chubby, I also developed an eating disorder which propelled my weight into the obese category before I hit my teens. As a result, I believe I developed PCOS which in conjunction with my excess eating gave my weight an extra push before I entered high school.
Anyway, looking back on my weight gain in my early years, I believe it is a bunch of genetic tendencies which I didn't fight nor did I probably know how to fight that pushed me to 300 lbs.
Of course 300 lbs wasn't my highest weight. I gained 30 lbs in 5 years of college, mostly a result of the lifestyle being distracted by college, not paying attention to my weight and continued excess eating. I ended up gaining 30 lbs after college within a year due to getting a car, lack of natural exercise and continued eating issues. It also took me 5 years before I would successfully start to lose weight even though I tried throughout those 5 years.
Personally, I don't like the "genetics make me fat" argument. I think it holds people back from losing weight. Why try if my own genetics are fighting against me? Genetics may make it tougher to lose weight, genetics may make gaining weight easier, but I think genetics don't make you fat all on their own.
You can't out-exercise poor eating habits.