In an inducible operon, the genes are

Options

A. Always expressed
B. Usually not expressed unless a single turns them on
C. Usually expressed unless a single turns them off
D. Never expressed

Answer

### Step-by-Step Solution:

1. **Understanding the Operon Concept**:
- An operon is a cluster of genes under the control of a single promoter. It consists of structural genes, a promoter, and an operator.

2. **Defining Inducible Operons**:
- Inducible operons are a type of operon that is usually in the "off" state. This means that the genes are not expressed under normal conditions.

3. **Role of Inducers**:
- The operon can be turned "on" when an inducer is present. The inducer is a molecule that binds to the repressor protein, preventing it from binding to the operator.

4. **Gene Expression in Inducible Operons**:
- In the presence of an inducer, the operon is activated, and the genes are expressed. This means that the genes are usually not expressed unless a signal (the inducer) turns them on.

5. **Conclusion**:
- Therefore, in an inducible operon, the genes are usually not expressed unless a signal turns them on. This leads us to the correct answer, which is option B.

### Final Answer:
In an inducible operon, the genes are usually not expressed unless a signal turns them on.

---