There is no such thing as "only genetic" or "only environmental" obesity. All organisms are both programmed genetically, and living in a particular environment. One cannot separate them in this way. Only if two people are put into the exact same environment and show different responses can one say that one is perhaps genetically predisposed to be different than the other. Each organism has a norm of reaction which specifies their possible response to a particular environment. Simply because one or more "obesity genes" have been located does not mean that humans who carry these will become definitively obese regardless of environmental intervention. It also does not mean that humans who do not express these genes cannot become obese.

As an example of this, gardeners know that particular plants usually grow in a certain way. A certain plant may grow to a certain height, flower, and possess particular characteristics like leaf shape, stem length, etc. This would show the norm of this plant's genetic code. However, since plants must be grown somewhere, the environment is crucial to how the plant expresses this genetic code. There is no "normal" environment for a plant, only environments that produce particular expressions. In sun the plant may be bushy and covered with flowers. In shade the plant may be pale, spindly, and maybe not even flower. We could say that the plant is genetically programmed to flower in the sun. But we could also say that the plant is genetically programmed to be spindly in the shade.

In terms of people, we could say that given caloric excess and sedentary lifestyle, person A is programmed to be obese. But we could also say that given caloric deficit and active lifestyle, person A is programmed to be fit. Perhaps person A requires fewer calories than person B to express this tendency to be obese, but person B could easily also become obese with caloric excess (whatever that means to their individual physiology) and sedentary lifestyle.