It is certainly a very difficult question to answer.
There are some genetic disorders that can predispose individuals for becoming obese: https://www.ncbi.nlm.nih.gov/pubmed/18230893
There are also evidence of epigenetic influences on obesity: http://www.nature.com/nrendo/journal/v12/n3/full/nrendo.2016.18.html
Typically each case of obesity is caused a combination of both genetics, epigenetics and lifestyle: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3873060/
However also keep in mind that obesity is a fairly recent problem and as such the genetic changes are likely minimal between the 1980 and 2008 populations sampled by WHO: http://www.who.int/gho/ncd/risk_factors/obesity_text/en/
A doubling in the prevalence of obesity in the two populations are most likely caused by changes in lifestyle and the availability of food, rather than genetic changes as the time frame is so short.
So to sum up: Morbid obesity caused by currently unfavorable genetics exists but is very rare. People can be genetically and epigenetically predisposed for developing obesity, which when combined with certain lifestyles can further increase the risk of becoming obese.