Provide an example of how gene regulation influencing phenotype in a simple model organism.

• A. In bacteria, lac operon expression in the presence of lactose enables lactose metabolism.
• B. In bacteria, ribosome production is unaffected by lactose.
• C. In bacteria, gene regulation has no effect on phenotype.
• D. In bacteria, lactose inhibits gene expression.

Answer: (A)

Gene regulation can shape phenotype by turning specific genes on or off in response to the environment, producing proteins that give the cell new capabilities. The lac operon in bacteria is the classic example: when lactose is available, the cell activates the genes needed to metabolize it, so enzymes and transport proteins are produced that enable lactose usage.

Mechanistically, in the absence of lactose a repressor sits on the operator and blocks transcription of the lac genes. When lactose is present, it is converted to a small molecule that binds the repressor, inactivating it. This releases the block, RNA polymerase transcribes the lac genes, and enzymes like beta-galactosidase and lactose permease are made, allowing the cell to import and break down lactose. The phenotype—ability to metabolize lactose—emerges directly from this regulatory control of gene expression.

The other statements don’t fit because they either deny regulatory effects on phenotype, claim regulation is irrelevant, or state the opposite of what actually happens (lactose inducing, not inhibiting, lac gene expression).