A human being suffering from Down syndrome shows trisomy of 21st chromosome. Mention the cause of this chromosomal abnormality.

Explanation

**Step-by-Step Text 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. **Chromosomal Composition**: In humans, there are normally 46 chromosomes arranged in 23 pairs. One chromosome of each pair is inherited from the mother (via the egg) and one from the father (via the sperm).

3. **Cause of Trisomy 21**: The primary cause of Down syndrome is a chromosomal abnormality known as non-disjunction. This occurs when the 21st chromosome fails to separate properly during cell division (meiosis) in the formation of the egg or sperm.

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