Number of chromosomes can increase or decrease due to

Options

A. Mutation
B. Genetic repetition
C. Nondisjunction
D. All the above

Answer

To answer the question about how the number of chromosomes can increase or decrease, we can break down the concepts involved:

### Step-by-Step Solution:

1. **Understanding Chromosome Number**:
- Chromosomes are structures within cells that contain DNA. Each species has a characteristic number of chromosomes. For example, humans have 46 chromosomes.

2. **Causes of Changes in Chromosome Number**:
- Chromosome number can change due to various genetic events. The question specifically asks about mutations that can lead to an increase or decrease in chromosome number.

3. **Types of Mutations**:
- **Genetic Repetition**: This refers to the duplication of a segment of DNA. While this can lead to more genetic material, it does not typically change the overall chromosome number.
- **Non-Disjunction**: This is a specific type of error that occurs during cell division (meiosis or mitosis) where homologous chromosomes or sister chromatids fail to separate properly. This can lead to gametes with an abnormal number of chromosomes.

4. **Impact of Non-Disjunction**:
- If non-disjunction occurs, it can result in gametes that have either one extra chromosome (n+1) or one less chromosome (n-1). When these gametes fuse during fertilization, the resulting zygote can have an abnormal number of chromosomes, leading to conditions such as Down syndrome (trisomy 21) or Turner syndrome (monosomy X).

5. **Conclusion**:
- Therefore, the correct answer to the question is that the number of chromosomes can increase or decrease primarily due to **non-disjunction**, which is the failure of homologous chromosomes or sister chromatids to separate during cell division.

### Final Answer:
The number of chromosomes can increase or decrease due to **non-disjunction**.

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