What evidence is there to suggest that delayed gratification is taught and learned and not genetic? Every evening I spend considerable time and energy trying to get my kids to eat their dinner, holding off treats such as chips or dessert. My experience was that my parents did the same. 

I have found over the course of my higher-education studies, that my ability to delay gratification was a big factor in my ability to complete degrees in Law and Computer Science. 

My question is: What evidence is there to suggest that delayed gratification is taught and learned and not genetic?

ie Did I learn delayed gratification or did I just have it? (I didn't seem to have it as a child at the dinner table). Now as a parent with my children at the dinner table, do I have a hope of teaching them delayed gratification?
whilst the nature-nurture issue in mammalian (and particularly human) behaviour is a perennially-fascinating topic, we have learned to be cautious about attempting to construct any strict dichotomies between learned and innate behaviour. suppose you had asked: "did I learn English or just have it?". 

successful delayed gratification most likely involves cognitively-mediated inhibition channeled via the frontal lobes. the behaviour must be learned, but it relies on a genetically encoded neural substrate. since it has been claimed (on the basis of longitudinal studies) that the degree of success in delayed gratification tasks persists over time, it may be interesting to ask a related question: if a person has a below-average ability to successfully delay gratification, to what extent is this due to poor teaching and to what extent is it based on neurological factors?