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 provided options and determine which phenomenon is specifically associated with population studies.

### Step-by-Step Solution:

1. **Understanding Population Studies**:
- Population studies involve examining the genetic variations and distributions of traits within a population. They help in understanding how traits are inherited and how they vary among individuals.

2. **Analyzing the Options**:
- **Multiple Alleles**: This phenomenon occurs when more than two alleles exist for a gene within a population. A classic example is the ABO blood group system. This concept can be studied in populations, but it is not exclusive to population studies.
- **Co-dominance**: This occurs when both alleles in a heterozygote are fully expressed, resulting in offspring with a phenotype that is neither dominant nor recessive. An example is a red flower crossed with a white flower producing a pink flower. This phenomenon can be observed in individual crosses and is not limited to population studies.
- **Pleiotropy**: This phenomenon occurs when a single gene influences multiple phenotypic traits. An example is a gene affecting skin color, which can influence various traits. Pleiotropy can be studied in individuals and does not require population-level analysis.
- **Phenotypic Variation**: This refers to the observable differences in traits among individuals in a population. It is a key focus of population studies as it examines how traits vary and are distributed in a population.

3. **Identifying the Correct Answer**:
- Among the options, **Phenotypic Variation** is the phenomenon that can only be fully understood and studied through population studies. It encompasses the variations observed in traits across a population, which is the essence of population genetics.

### Conclusion:
The phenomenon that can be found only when population studies are conducted is **Phenotypic Variation**.