It can be argued that gratification delay is more psychology related than genetics. And while it can be found a pattern on genes, probably it will be falsified like happened before with these kind of experiments.
Experiments require resources which are limited. And after the falsification of intelligence genes, it makes sense to find a psychological cause for delay of gratification first, and test other logical explanations second.
Edit: I was wrong. Reading Wikipedia article (https://en.wikipedia.org/wiki/Heritability_of_IQ), it seems intelligence is related to both genes and enviroment.