Sudden addition or deletion of genes by chance in a small population is called
(a) Sewall wright effect
(b) Mutation
(c) Gene flow
(d) Gene migration

Options

A. Sewall wright effect
B. Mutation
C. Gene flow
D. Gene migration

Answer

To solve the question, "Sudden addition or deletion of genes by chance in a small population is called," we need to analyze each of the options provided:

1. **Understanding the Question**: The question is asking about a phenomenon where there is a sudden change in the genetic makeup of a small population due to random events.

2. **Analyzing the Options**:
- **(a) Sewall Wright Effect**: This term refers to the genetic drift that occurs in small populations, where random events can lead to significant changes in gene frequencies. It is characterized by the random addition or deletion of genes, which aligns with the question.
- **(b) Mutation**: This refers to a change in the DNA sequence, which can occur in any population size but does not specifically address the randomness in a small population context.
- **(c) Gene Flow**: This is the transfer of genes between populations, typically through migration, and does not pertain specifically to random changes within a single small population.
- **(d) Gene Migration**: Similar to gene flow, this involves the movement of genes between populations and does not apply to the sudden changes within a small population.

3. **Conclusion**: Given the definitions and the context of the question, the correct answer is **(a) Sewall Wright Effect**. This effect specifically describes the random fluctuations in gene frequency that can occur in small populations due to chance events.

### Step-by-Step Solution:
1. Read and understand the question.
2. Analyze each option provided.
3. Identify that the Sewall Wright Effect describes genetic drift in small populations due to chance events.
4. Conclude that the correct answer is (a) Sewall Wright Effect.