Answer:
Enzyme
Step-by-step explanation:
Enzymes are the bio-catalyst that serve to accelerate the metabolic reactions occurring in a living cell. All the metabolic reactions of organisms are catalyzed by enzymes. Each metabolic reaction is catalyzed by specific enzyme. Absence of that particular enzyme leads to absence of the respective product. Since genes code for protein which regulate the specific trait, absence of enzyme does not allow the expression of that trait.
Example: Absence of enzymes for chlorophyll synthesis would lead to absence of chlorophyll from leaves of the green plants.