Aneuploidy arises through

Options

A. disjunction
B. nondisjunction
C. inversion
D. duplication

Answer

### Step-by-Step Text Solution

1. **Definition of Aneuploidy**:
- Aneuploidy is defined as the presence of an abnormal number of chromosomes in a cell. This means that instead of the normal diploid number (for humans, 46 chromosomes), there can be one or more chromosomes missing or extra.

2. **Understanding Chromosome Number**:
- In humans, the typical chromosome number is 46, which consists of 23 pairs. Aneuploidy can result in having 45 (monosomy) or 47 (trisomy) chromosomes.

3. **Origin of Aneuploidy**:
- Aneuploidy arises from errors during cell division. Specifically, it originates from a failure in the proper segregation of chromosomes during meiosis or mitosis.

4. **Concept of Non-Disjunction**:
- The primary mechanism that leads to aneuploidy is called non-disjunction. Non-disjunction occurs when homologous chromosomes or sister chromatids fail to separate properly during cell division.

5. **Consequences of Non-Disjunction**:
- When non-disjunction occurs, one daughter cell may receive an extra chromosome (resulting in trisomy), while the other daughter cell may be missing that chromosome (resulting in monosomy). This can lead to various genetic disorders.

6. **Common Types of Aneuploidy**:
- The most common aneuploidies in humans involve chromosomes 21 (Down syndrome), 18 (Edwards syndrome), and 13 (Patau syndrome). These conditions often result from non-disjunction events.

7. **Conclusion**:
- Therefore, the correct answer to the question "Aneuploidy arises through" is non-disjunction, which refers to the failure of chromosomes to separate properly during cell division.