Klinefelter's syndrome results from the fusion of

Options

A. an X egg and a YY sperm.
B. an XY egg and an X sperm.
C. an XX egg and a Y sperm.
D. an XX egg and a YY sperm.

Answer

### Step-by-Step Solution:

1. **Understanding Klinefelter's Syndrome**: Klinefelter's syndrome is a genetic condition that occurs in males and is characterized by the presence of an extra X chromosome. The typical chromosomal composition for Klinefelter's syndrome is 47,XXY instead of the normal 46,XY.

2. **Chromosomal Composition**: The chromosomal makeup of an individual with Klinefelter's syndrome is 44 autosomes and 2 sex chromosomes (XXY). This means that there are 44 chromosomes that are not sex chromosomes (autosomes) and 3 sex chromosomes (XXY).

3. **Fusion of Gametes**: Klinefelter's syndrome arises from the fusion of an egg and a sperm. The egg contributes one X chromosome, and the sperm contributes one X and one Y chromosome, leading to the XXY composition.

4. **Analyzing the Options**:
- **Option 1**: N-Egg, N-A-Y-Y-Sperm - This suggests the presence of two Y chromosomes which is not characteristic of Klinefelter's syndrome.
- **Option 2**: XY-Egg - This is incorrect as eggs do not carry Y chromosomes; they only carry X chromosomes.
- **Option 3**: XX-Egg and Y-Sperm - This is the correct combination that results in Klinefelter's syndrome (XXY).
- **Option 4**: 2X from egg and 2Y from sperm - This combination would not occur in Klinefelter's syndrome.

5. **Conclusion**: The correct answer is that Klinefelter's syndrome results from the fusion of an XX egg and a Y sperm, leading to the chromosomal composition of 44 + XXY.