Down's syndrome or Mongoloid idiocy (Mongolism) is due to

Options

A. trisomy off X chromosome
B. trisomy of 21st chromosome
C. nullisomy of a chromosome
D. monosomy of an autosome

Answer

**Step-by-Step Solution:**

1. **Understanding Down's Syndrome**: Down's syndrome, also known as Mongoloid idiocy or Mongolism, is a genetic disorder characterized by a combination of physical and intellectual disabilities.

2. **Identifying the Cause**: The primary cause of Down's syndrome is related to chromosomal abnormalities. Specifically, it is caused by the presence of an extra copy of chromosome 21.

3. **Defining Trisomy**: The condition where there are three copies of a chromosome instead of the usual two is known as "trisomy." In the case of Down's syndrome, this is referred to as trisomy 21.

4. **Understanding Chromosomal Pairs**: Normally, humans have 23 pairs of chromosomes (46 in total). In Down's syndrome, an individual has three copies of the 21st chromosome, leading to a total of 47 chromosomes.

5. **Mechanism of Occurrence**: This extra chromosome typically arises due to nondisjunction during cell division, which can occur in either the egg or sperm cell. Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division.

6. **Symptoms of Down's Syndrome**: Individuals with Down's syndrome may exhibit various symptoms, including delayed development, short stature, and congenital heart defects.

7. **Conclusion**: Therefore, the correct answer to the question is that Down's syndrome or Mongoloid idiocy is due to the trisomy of the 21st chromosome.

**Final Answer**: Down's syndrome is due to the trisomy of the 21st chromosome.

---