The gene, which affects many phenotypes.

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a type of gene that affects multiple phenotypes. This means we are looking for a gene that can influence various traits or characteristics in an organism.

2. **Identifying the Concept**: The concept that describes a gene affecting multiple phenotypes is known as "pleiotropy." Pleiotropic genes are those that can have multiple effects on different traits.

3. **Defining Pleiotropic Genes**: Pleiotropic genes are genes that, when mutated or expressed, can lead to changes in multiple traits. This is because a single gene can influence various biological pathways or processes.

4. **Examples of Pleiotropic Effects**:
- **Sickle Cell Anemia**: This genetic condition is caused by a mutation in a single gene (the hemoglobin gene) but affects multiple systems in the body, leading to symptoms such as pain, anemia, and increased risk of infections.
- **Cystic Fibrosis**: Another example where a mutation in the CFTR gene affects the respiratory, digestive, and reproductive systems.
- **Phenylketonuria (PKU)**: A metabolic disorder caused by a mutation in a single gene that affects multiple systems, leading to intellectual disability and other health issues.

5. **Conclusion**: Therefore, the answer to the question is that the type of gene which affects many phenotypes is called a **pleiotropic gene**.

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