When two unrelated individuals or lines are crossed, the performance of `F_(1)` hybrids is often superior to both its parents. The phenomenon is called

Options

A. Metamorphosis
B. Heterosis
C. Transformation
D. Splicing

Answer

### Step-by-Step Solution:

1. **Understanding the Concept**: The question refers to a phenomenon observed when two unrelated individuals or lines are crossed, resulting in the F1 hybrids exhibiting superior traits compared to their parents. This concept is crucial in genetics and plant/animal breeding.

2. **Identifying the Process**: When two genetically distinct individuals are crossed, the resulting offspring (F1 hybrids) often show enhanced performance, which can include better growth, yield, or other desirable traits. This enhancement is due to the combination of diverse genetic traits from both parents.

3. **Defining the Phenomenon**: The phenomenon where the F1 hybrids outperform both parents is known as **heterosis**. Heterosis is often referred to as hybrid vigor, where the hybrid exhibits improved or superior biological qualities.

4. **Example of Heterosis**: A classic example of heterosis is the cross between a male donkey and a female horse, resulting in a mule. The mule inherits the strength of the horse and the endurance of the donkey, showcasing the benefits of hybrid vigor.

5. **Conclusion**: Therefore, the answer to the question is that the phenomenon is called **heterosis**.