A pleiotropic gene is

Options

A. `I^(A)`
B. `Hb^(S)`
C. `Hb^(A)`
D. `I^(B)`

Answer

### Step-by-Step Solution

1. **Understanding the Definition of a Pleiotropic Gene**:
A pleiotropic gene is defined as a single gene that influences multiple phenotypic traits. This means that one gene can have several effects on different characteristics of an organism.

2. **Contrast with Monogenic Traits**:
In contrast to pleiotropic genes, some genes are responsible for controlling a single trait or phenotype. For example, a gene that determines plant height (like the capital T gene for tallness) is an example of a gene affecting a single phenotype.

3. **Mechanism of Pleiotropy**:
The underlying mechanism of pleiotropy often involves a gene that affects multiple metabolic pathways. This means that the gene's product (like a protein) can interact with various biological processes, leading to multiple observable traits.

4. **Examples of Pleiotropic Genes**:
- **Phenylketonuria (PKU)**: A genetic disorder caused by a mutation in a gene that affects the metabolism of phenylalanine, leading to multiple health issues.
- **Sickle Cell Anemia (HBS)**: A classic example of pleiotropy where a single point mutation in the hemoglobin gene causes red blood cells to become sickle-shaped, leading to various complications such as jaundice, spleen damage, and brain damage.

5. **Identifying Pleiotropic Genes**:
In the context of the question, the sickle cell anemia gene (HBS) is identified as a pleiotropic gene because it affects not only the shape of red blood cells but also causes a range of secondary health issues due to its impact on multiple systems in the body.

6. **Conclusion**:
The correct answer to the question is that a pleiotropic gene is one that exhibits multiple phenotypic effects, with the HBS gene for sickle cell anemia being a prime example.