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.
I want to praise you for taking the time to admit that you were wrong about a topic you didn't know much about. It is something that is seen far too infrequently in this world.