Understanding Genotype
The term "genotype" refers to the genetic constitution of an individual organism. It encompasses all the alleles an organism possesses, which can determine various traits. Here's a detailed breakdown:

Definition of Genotype
- The genotype is the **specific genetic makeup** of an individual, represented by the combination of alleles (variant forms of a gene).
- It includes both **dominant and recessive alleles** that an individual inherits from their parents.

Importance of Genotype
- Genotype plays a critical role in determining **phenotypic traits**, which are the observable characteristics of an organism, such as eye color, height, and susceptibility to diseases.
- Understanding genotype helps in **studying inheritance patterns**, as it elucidates how traits are passed down through generations.

Genotype vs. Other Terms
- **Genetic composition of the individual (Option A)**: This is correct because it refers to the entire genetic makeup of the organism, including all genes and alleles.
- **Genetic composition of the germ cell (Option B)**: This is too specific; while germ cells do carry genetic information, genotype refers to the whole organism.
- **Genetic composition of plastids (Option C)**: Plastids are organelles and do contain their own DNA, but this is not what is meant by genotype.
- **Genetic composition of an organ (Option D)**: This is also too localized and does not encompass the entire genetic identity of the organism.

Conclusion
In summary, the correct answer is **option A**, as genotype accurately describes the complete genetic makeup of an individual organism, which profoundly influences its development and traits. Understanding genotype is essential for genetics, breeding, and evolutionary studies.