Occasionally, a single gene may express more than one effect. The phenomenon is called

Options

A. multiple allelism
B. mosaicism
C. pleiotropy
D. polygeny

Answer

To solve the question, "Occasionally, a single gene may express more than one effect. The phenomenon is called," we need to analyze the options provided and identify the correct term that describes this genetic phenomenon.

### Step-by-Step Solution:

1. **Understanding the Question**:
- The question is asking for a term that describes a situation where one gene has multiple effects or influences on different traits.

2. **Analyzing the Options**:
- **Option 1: Multiple Allelism**:
- This term refers to a situation where more than two alleles exist for a single gene, but it does not imply that one gene affects multiple traits. Therefore, this option is incorrect.

- **Option 2: Mosaicism**:
- Mosaicism refers to the presence of two or more genetically different cell lines in the body of an individual. This does not relate to a single gene affecting multiple traits. Hence, this option is also incorrect.

- **Option 3: Pleiotropy**:
- Pleiotropy is the phenomenon where a single gene influences multiple phenotypic traits. This directly matches the description in the question, making this the correct answer.

- **Option 4: Polygenic**:
- Polygenic inheritance involves multiple genes contributing to a single trait. This is the opposite of what the question is asking, as it does not refer to a single gene affecting multiple traits. Therefore, this option is incorrect.

3. **Conclusion**:
- After analyzing all the options, we conclude that the correct answer is **Pleiotropy** (Option 3), as it accurately describes the phenomenon where one gene can express more than one effect.

### Final Answer:
The phenomenon is called **Pleiotropy**.