Usually it is though, so I don’t agree.
Some years ago a scientific study was published after analyzing thousands of test people, and the study indeed seem to suggest that genetics played a role.
But not as much one would expect.
The maximum difference in weight between people of the same gender and height, with similar diets and sports curriculum, was something like 17.6 lb (8 kg).
So although the population was large, there was not one example of (say): Betty is 5′10′’ tall while Samantha also is. They eat the same stuff every day, and run 2 hours per week. Betty weighs 250 lb and Samantha 120 lb.
Nope.
Same gender, height, diet and sport means essentially same weigh — apart from a possible rather small difference due to genetics (and other factors).
And that’s it.
Mind you: some people can suffer from some kind of condition which heavily increases weight, but usually that’s not the case (such conditions are rare).
Ricky Gervais (himself having been obese for a large part of his life) said in one of his shows:
“You don’t get fat behind your own back.”
And he’s right.
Of course, rather than genetics, family can be a very bad factor in this story.
If your mother is very obese, the probability of you or one of your siblings becoming obese (even as a kid) is pretty high, because kids often “inherit” the eating habits of parents (and in particular, mothers).
And a large part of obese kids, also are obese as adults.
(I had a girlfriend once who was laughed at by her own mother because she was “too skinny.” But actually it was the mother who was too fat, and she did not like the contrast. Family relations can become poisonous, especially between mothers and daughters.)
And to be totally honest: I never said I am healthy myself.
I know that I drink way too much, and in the last couple of years do little to no sports, and that is not good.
But I don’t blame the genes.