There are at least 17 gene variants responsible for increasing your body mass index which can predispose you to putting on 2-4 times the level of fat that thinner people do on the same amount of food and exercise.
You can cut this genetic predisposition by 40% according to researchers particularly during adolescence by exercising at well above average levels for example 3-4 hours of exercise every day or at least 1-2 hours of very vigorous exercise every day.