Playing devil's advocate: they may say that the genes were always there, but they were just a pre-disposition to overconsumption in the presence of a hypercaloric enviornment (which has only recently existed). HOWEVER, this does not excuse someone from becoming a hambeast! Just like a genetic predisposition to alcoholism doesn't excuse someone as the OP pointed out. Certain aspects of life may be harder/easier for all of us depending on the hand we're dealt by our genetics/upbringing, but it's NEVER an excuse to give up and be a total burden to society.