The brain has been ascribed many abilities it cannot possibly possess from learning, to seeing, to understanding, to knowing, etc. This is the mereological fallacy writ large. Ascribing attributes/characteristics/behaviors to a part or parts of a thing that can only (logically) be applied to the whole thing. Only of a (mostly) whole human person (and some non human animals) with a (mostly) functioning nervous system including a brain can we say they are capable of doing any of those things. Never forget that a brain divorced from a body is a lump of grey matter capable of nothing save a few basic cellular processes and possibly some low level electrochemical (nerve) activity, for a short period of time at least. It is no doubt the case that the brain has some role in gene expression, however, I do not think that is what you are suggesting with your comment.
You believe that the conscious mind (our conscious beliefs/awareness) have some (“a powerful”) influence on gene expression. This is a highly controversial and probably not testable hypothesis. That said just because a hypothesis is presently untestable does not by itself mean that it cannot be the case. Another good example of a presently untestable hypothesis that many people believe is true is the simulation hypothesis. Though it is highly improbable for a host of reasons, there are at least (moderately strong) logical and mathematical arguments in favor of it. In the case of conscious mind influence on gene expression we do not have even that. Instead we mostly have pithy sayings like “mind over matter” or “think yourself thin” combined with first person testimonials that are impossible to verify.
I happen to believe that the conscious mind may have some influence on gene expression but it is only at the margins, and the effect is probably not significant enough to warrant much effort from a personal perspective. In contrast, the mind has considerable and virtually unchecked power and control over many of the environmental factors which we know are key in staying physically and mentally healthy such as not smoking, exercising, eating right, etc. Rather than counting on a hypothetical mind/body/gene expression link to save us perhaps we should focus more on the mind/mind/habits/actions link that for sure exists and use it to exert more control over our bodies in ways we know are beneficial.
And I almost forgot, thanks for reading and commenting!! It is very much appreciated.