Assertion: Gene expression is a molecular mechanism by which a gene expresses a phenotype.
Reason: Structural genes are controlled by control genes.

Options

A. If both assertion and Reason are true and the Reason is the correct explanation of the assertion.
B. If both assertion and Reason are true but the reason is not the correct explanation of the assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false.

Explanation

To solve the assertion-reason question, we will analyze both the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states: "Gene expression is a molecular mechanism by which a gene expresses a phenotype."

- **Explanation**: This statement is true. Gene expression refers to the process by which information from a gene is used to synthesize a functional gene product, often proteins. These proteins play a crucial role in determining the phenotype of an organism, which includes observable traits such as physical characteristics, behavior, and biochemical properties.

### Step 2: Analyze the Reason
The reason states: "Structural genes are controlled by control genes."

- **Explanation**: This statement is false. Structural genes are not controlled by "control genes." Instead, they are regulated by regulatory genes. Regulatory genes produce proteins that can enhance or inhibit the expression of structural genes. In prokaryotic organisms, this regulation often occurs through operons, where regulatory genes control the transcription of structural genes.

### Step 3: Conclusion
Since the assertion is true and the reason is false, the correct answer is that the assertion is true but the reason is false.

### Final Answer
The correct option is C: Assertion is true, but the reason is false.

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