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A human being suffering from Down's Synd...

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

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Due to non-disjunction, `21^(st)` pair of chromosomes fail to separate during oogenesis. Therefore, the egg possesses 24 chromosomes instead of 23. When the egg fuses with a sperm, the zygote will have three copies of chromosome 21 causing trisorny.
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