The gene thing is true to a certain extent
No, no its not. The human body is essentially a machine, there are certain genetic or environmental factors that influence the way that it works but at a base level, if you consume more calories than you use you will put on weight. If you consume less calories than you burn, or burn more than you consume, you will lose weight. It really is as simple as that.
Like I said, true to a certain extent, there is still a lot of research in this field and cannot be discounted with a swoop. I consider my self lucky that I can eat a lot on not put on weight, but there are people who eat less than me and are more active and are much heavier! Basically the research shows that your body will strive to keep to what t thinks is the ultimate weight.
http://www.timesonline.co.uk/tol/news/u ... 647517.ece
clicky links above
edit: just to point out, for much of my teens I strived to put on weight as i was super skinny! I ate a lot but no weight came, and i'm still quite thin.
Put me in a room with another person of similar build, we eat the same what will happen? we each put on the same weight? maybe, maybe not, cannot be quantified theoritically because each of our bodies is different and what makes us different is our genes.
Now lifestyle and knowing our bodies, eating healthy and excersing is a different argument.