A human being suffering from Down's Syndrome shows trisomy of `21^(st)` chromosome. Mention the cause of this chromosomal abnormality.

Explanation

**Step-by-Step Solution:**

1. **Understanding Down's Syndrome**: Down's Syndrome, also known as Trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. This results in a total of 47 chromosomes instead of the normal 46.

2. **Identifying the Chromosomal Abnormality**: The specific chromosomal abnormality in Down's Syndrome is the trisomy of chromosome 21. This means that individuals with Down's Syndrome have three copies of chromosome 21.

3. **Cause of Trisomy 21**: The primary cause of this chromosomal abnormality is a process called **nondisjunction**. Nondisjunction occurs when the paired chromosomes fail to separate properly during cell division (meiosis).

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