This is not completely understood yet, but recent research documents how stress can alter the expression of the acetylcholinesterase gene to generate at least 3 alternative proteins that are implicated in a wide variety of normal mind-body functions, as well as pathologies. These range from early embryological development, plasticity of the brain in adulthood, post-traumatic stress disorder (PTSD), and stress-associated dysfunctions of the central nervous, endocrine, and immune systems, to age-related neuropathologies. Such stress-induced alternative gene splicing is proposed as a major mind-body pathway of psychosocial genomics.
As you can see, even though one gene=one protein, some other means can change this so that the gene is now going to produce an altenative protein.