Here is a quote from a paper published in 1990 (over 25 years ago!!!) by a H. F. Nijhout in an article entitled Metaphors and the Role of Genes in Development published in BioEssays:
“When a gene product is needed,
a signal from its environment,
not an emergent property of the gene itself, activates expression of that gene.”
A gene “product” mentioned in line 1 above represents a protein. In line 2 it states that the “signals arise from the environment.”
In line 3, the emphasis is that genes do not self-actualize, that is they do NOT control their own activity.
This statement defines what is meant by the science of epigenetics: it includes the molecular mechanisms by which genes are activated by environmental signals.
Since the 1990s, we now know that epigenetic signals can also modulate the amount of activity a gene will express as well as the exact nature of the protein created from a gene (as mentioned, through epigenetic mechanisms a given gene can be used to create over 2000 different protein products!)