Down's syndrome in humans is due to

Options

A. Two 'Y' chromosomes
B. Three 'X' chromosomes
C. Three copies of chromosome 21
D. Monosomy

Answer

**Step-by-Step Solution:**

1. **Understanding Down's Syndrome**: Down's syndrome is a genetic disorder that occurs due to an abnormal number of chromosomes in a person's cells. It is specifically characterized by the presence of an extra copy of chromosome 21.

2. **Chromosomal Basis**: Normally, humans have 46 chromosomes arranged in 23 pairs. Each pair consists of one chromosome from each parent. In Down's syndrome, there are three copies of the 21st chromosome instead of the usual two. This condition is known as trisomy 21.

3. **Aneuploidy Explanation**: The condition arises from a process called aneuploidy, which is the gain or loss of chromosomes due to the failure of proper segregation during cell division (either meiosis or mitosis). This failure can occur during the anaphase stage of cell division.

4. **Identifying the Cause**: The specific cause of Down's syndrome is the nondisjunction of chromosomes during cell division, leading to the formation of gametes with an abnormal number of chromosomes. When one of these gametes participates in fertilization, the resulting zygote has an extra chromosome 21.

5. **Symptoms and Characteristics**: Individuals with Down's syndrome may exhibit several physical traits, such as a flat back of the head, broad flat face, and specific hand features like palm creases and loops on fingertips. They may also experience developmental delays and congenital heart defects.

6. **Conclusion**: The answer to the question is that Down's syndrome in humans is due to the presence of three copies of the 21st chromosome, which is a result of aneuploidy.

**Final Answer**: Down's syndrome in humans is due to the presence of three copies of the 21st chromosome (trisomy 21).

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