People with Kleinfelter's syndrome have 47 chromosomes, including three sex chromosomes (XXY). What is the term to describe the aberration that occurs during meiosis that results in abnormal chromosome number?

Options

A. Crossing over
B. Non-disjunction
C. Independent assortment
D. Pairing of homologous chromosome.

Answer

To solve the question regarding the aberration that occurs during meiosis leading to Klinefelter's syndrome (XXY), we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Klinefelter's Syndrome**:
- Klinefelter's syndrome is characterized by the presence of an extra X chromosome in males, resulting in a total of 47 chromosomes (XXY).

2. **Identifying the Cause of Chromosomal Aberration**:
- The abnormal number of chromosomes is due to an error in meiosis, the process that produces gametes (sperm and egg cells).

3. **Reviewing the Options**:
- The options provided are:
- A) Crossing over
- B) Non-disjunction
- C) Independent assortment
- D) Pairing of homologous chromosomes

4. **Analyzing Each Option**:
- **A) Crossing Over**: This is the exchange of genetic material between homologous chromosomes during prophase I of meiosis. It does not result in an abnormal number of chromosomes.
- **B) Non-disjunction**: This refers to the failure of homologous chromosomes or sister chromatids to separate properly during meiosis. This can lead to gametes with an abnormal number of chromosomes (e.g., an egg with two X chromosomes).
- **C) Independent Assortment**: This principle states that alleles for different traits segregate independently of one another during gamete formation. It does not cause an abnormal number of chromosomes.
- **D) Pairing of Homologous Chromosomes**: This occurs during meiosis but does not directly lead to an abnormal chromosome number.

5. **Conclusion**:
- The correct term that describes the aberration leading to Klinefelter's syndrome is **Non-disjunction**. This is because it results in gametes that have an abnormal number of chromosomes, which can lead to the XXY condition when fertilization occurs.

### Final Answer:
**Non-disjunction**