[iguy]

It's worth noting that there are two uses of gene here, or at least of "genetic".

One of which is roughly DNA that codes for a protein, or controls such, and indeed the causal mechanisms from there to phenotype can be extremely complicated and hard to figure out.

In the context of "variation between people" there's another meaning, of just whatever it is that causes inheritance. We could say many things about this (e.g. from twin studies) even before we'd discovered DNA, and non-DNA routes (like if most babies get some microbes from their mothers) would still count.