What term is used to describe the gene-to-gene interaction in which the action of one gene modifies the expression of a different gene?

Epistasis is the term used to describe the gene-to-gene interaction in which the action of one gene modifies the expression of a different gene. Epistasis occurs due to the relationship between or within genes which leads to different non-linear effects.

there really is life in the deepest part

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.