Which of the following phenomenon can be found only when population studies are conducted ?

Options

A. Multiple allelism
B. Codominance
C. Incomplete dominance
D. Pleiotropy

Answer

To solve the question "Which of the following phenomenon can be found only when population studies are conducted?", we will analyze the options provided and identify the phenomenon that is specifically related to population studies.

### Step-by-Step Solution:

1. **Understanding the Question**: We need to determine which phenomenon is exclusively observed in population studies. Population studies often involve examining genetic variations and traits within a group of individuals.

2. **Analyzing the Options**:
- **Multiple Allelism**: This phenomenon occurs when there are more than two alleles for a genetic trait within a population. A classic example is the ABO blood group system, where multiple alleles (A, B, O) determine the blood type. This is indeed a phenomenon observed in population studies.

- **Co-dominance and Incomplete Dominance**: These terms refer to how alleles express themselves in heterozygous individuals. Co-dominance means both alleles are fully expressed (e.g., AB blood type), while incomplete dominance results in a blend of traits (e.g., red and white flowers producing pink). While these concepts can be studied in populations, they are not exclusive to population studies.

- **Pleiotropy**: This refers to a single gene affecting multiple traits. An example is skin coloration, where one gene can influence various aspects of skin color. Pleiotropy can be observed in individuals and is not limited to population studies.

3. **Conclusion**: Among the options, multiple allelism is the phenomenon that is specifically related to population studies. It involves the presence of multiple alleles within a population and is best illustrated by the ABO blood group system.

### Final Answer:
The correct option is **Multiple Allelism**.