How far can obesity and weight gain be down to genetics?

When I was younger, people who ate more than others put on more weight. Those who were weight or figure conscious ate less and healthier, those who dodn't give a damn ate more andfattier foods. They became fat, and no-one disputed this.
Now we are told it is largely genetic, that it is wrong to criticise someone's eating habits or attribte size to diet, as these people have genetic problems.
Even if that is so, it is surely just like having an allergy - you have to use willpower to eat less than you may like to.
One never hears of these casesof genetic obesity in places like Africa, the Himalayas or Bangladesh, only here, in countries where we have more than enough of everything.
Can somebody please clarify?

I think genetics gets mixed up with the family's taste and way of eating. In most fat families that I know no one cooks the eat fast food, and when they do cook its salty filled with fat, grease and sugar. The kids grow up on this stuff, so for generations and generations you'll have a family who is accustomed to liking the fatty salty food they grew up on.

CDC article below:

http://www.cdc.gov/Features/Obesity/

Genetics. It has been shown that children with obese parents are more likely to be obese. But is it for genetic or environmental reasons? One estimate says that heredity contributes between 5 and 25 percent of the risk for obesity.4 The remaining risk is attributed to environmental and behavioral factors. Others believe that genetics may play a bigger role. Regardless, the interrelationship between genetics and the environment is clear: Parents provide genes, role models, and food.

4. Bouchard C, Perusse L. Genetic aspects of obesity. Annals of the New York Academy of Sciences 1993; 699:26-35.