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 Syndrome**:
- Down syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21, leading to a total of three copies instead of the usual two. This condition is known as trisomy 21.

2. **Identifying the Type of Chromosomal Abnormality**:
- Down syndrome is classified as an aneuploidy, which refers to a condition where there is an abnormal number of chromosomes in a cell. In this case, there is a gain of an extra chromosome.

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