What you're referring to - the impact of the environment on genetic effects of a trait, is typically referred to as a "Gene-by-Environment" effect (or GxE). Practically speaking, this involves a change of model specification from expression ~ genotype + covariates to expression ~ genotype + condition + genotype*condition + covariates and examining the coefficient on the cross-term.

However, genetic effects in different environments need to be understood in the context of phenotypic differences across environments - if the phenotype has a different variance in the two conditions, then the same absolute genetic effect is a stronger relative genetic effect. I would say the frameworks for thinking about this are still under development; but a fantastic experimental paper that clearly elucidates several principles can be found here: https://www.nature.com/articles/s41467-020-19131-y