A syndrome which occures due to illegitimate crossing over is

Options

A. Klinefelter's syndrome
B. Familial Down's syndrome
C. Turner's syndrome
D. Patau syndrome

Answer

### Step-by-Step Solution:

1. **Understand the Concept of Illegitimate Crossing Over**:
- Illegitimate crossing over, also known as reciprocal translocation, refers to the exchange of genetic material between non-homologous chromosomes. This is different from the normal crossing over that occurs between homologous chromosomes during meiosis.

2. **Define Reciprocal Translocation**:
- Reciprocal translocation is a chromosomal abnormality where segments from two different chromosomes break and swap places. This results in a rearrangement of genetic material.

3. **Identify the Chromosomal Abnormality**:
- In humans, a common example of a syndrome caused by this type of chromosomal abnormality is Down syndrome, which is associated with an extra copy of chromosome 21.

4. **Link Down Syndrome to Illegitimate Crossing Over**:
- In familial Down syndrome, an extra copy of chromosome 21 can attach to another chromosome (for example, chromosome 14) due to reciprocal translocation. This results in the presence of three copies of chromosome 21 (trisomy 21), leading to the characteristics of Down syndrome.

5. **Conclusion**:
- Therefore, the syndrome that occurs due to illegitimate crossing over is known as **Familial Down Syndrome**.