Can environmental pressures affect genes in one generation?
Yes. Environmental pressures that cause a change in allele frequency will always produce changes in a single generation. If the environmental pressure is present for more than one generation, it will affect changes in allele frequency over a longer time span.
I'd encourage you to look at this set of case studies from UC Berkeley's Understanding Evolution
To clarify what "pressure" means in the context of evolution. Selective pressure, or evolutionary pressure, as wikipedia describes it, is anything that changes the reproductive success of an organism. It is, by definition, something that has an effect in one generation. Selective pressure causes organisms with certain heritable characteristics (and certain alleles) to reproduce at a different rate from organisms with other heritable characteristics (other alleles). The pressure selects for certain alleles.
Considering the comments, it sounds like you're describing a disorder (psychological ailment, e.g., PTSD, GAD), which may have a heritable diathesis, and an environment (trauma) that causes the expression of that disorder. To consider that environment to be selective pressure, it would need to be present at a population level and cause individuals with that heritable diathesis to reproduce at a different rate than individuals without it. If it did, you would see changes in allele frequency over one generation. Indeed there is no way for selective pressure to cause a change in allele frequency without it occurring in a single generation. I'm not aware of any study that has demonstrated this to be the case for any anxiety disorder.