In an inducible operon, the genes are

Options

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

Explanation

### Step-by-Step Text Solution:

1. **Understanding Inducible Operons**: An inducible operon is a type of operon that is typically off but can be turned on in response to an inducer molecule. This means that the genes within the operon are not expressed under normal conditions.

2. **Default State of Inducible Operons**: In the default state, the genes of an inducible operon are usually not expressed. This is because a repressor protein binds to the operator region of the operon, preventing transcription.

3. **Role of Inducers**: When an inducer molecule is present, it binds to the repressor protein, causing a conformational change that prevents the repressor from binding to the operator. This allows RNA polymerase to access the promoter and initiate transcription of the genes.

4. **Conclusion**: Therefore, the correct answer to the question is that in an inducible operon, the genes are usually not expressed unless a signal (inducer) turns them on.

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

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