In Down's syndrome, Karyotyping has shown that the disorder

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In Down's syndrome, Karyotyping has shown that the disorder is associated with trisomy of chromosome number-21 usually due to

1. non-disjunction during egg-cell formation
2. non-disjunction during sperm-cell formation
3. non-disjunction during formation of egg-cells and sperm-cells
4. addition of extra chromosome during mitosis of the zygote

Correct Option: C

Down’s syndrome caused by trisomy 21, arises due to an occassional non disjunction during meiosis when a gamete comes to possess an extra chromosome (n+1). Fusion with a normal gamete [n + (n+1)] produces trisomy.