You mentioned dopaminergic neurotransmission, specifically.

Notwithstanding that you've shared no compelling source (and I expect none), this is redundant - I like the taste of dark roast, the validity of that sentiment isn't contingent on genes. By no objective measure is one roast better tasting beyond mere experience. It's basically irrelevant whether I do possess those genes or not. Whether we like ANYTHING depends on our genes, so to say "you only like x because of genes" is a moot point - if entire cultures are primarily drinking dark, who's the real genetic outlier?