The substance which causes a defineite change in genes is called

Options

A. Mutagen
B. Toxin
C. Cytotoxin
D. Alkaloid

Answer

To solve the question "The substance which causes a definite change in genes is called," we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question is asking for a specific term that describes a substance that causes changes in genes.

2. **Identify Key Terms**: The key terms in the question are "substance," "definite change," and "genes." This indicates that we are looking for a term related to genetics.

3. **Analyze the Options**:
- **Mutagen**: A substance that causes mutations or changes in the DNA sequence of genes.
- **Toxin**: A poisonous substance that can harm organisms but does not specifically refer to changes in genes.
- **Cytotoxin**: A type of toxin that specifically targets and kills cells, not necessarily related to gene changes.
- **Alkaloid**: A class of naturally occurring organic compounds that mostly contain basic nitrogen atoms; while some alkaloids can act as mutagens, not all are directly related to gene changes.

4. **Determine the Correct Answer**: Based on the definitions:
- The term that best fits the description of a substance that causes definite changes in genes is **mutagen**. Mutagens are known for their role in inducing mutations in the genetic material.

5. **Final Answer**: Therefore, the correct answer is **mutagen**.