Down's syndrome in humans is due to

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(B) : Down's syndrome is the trisomy of the $21$st chromosome in humans.Down's syndrome is characterized by short stature,warty skin,protruding tongue

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three copies of chromosome $21$

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(B) : Down's syndrome is the trisomy of the $21$st chromosome in humans.Down's syndrome is characterized by short stature,warty skin,protruding tongue,slanting eyes,and folded eyelids.The affected person's face presents a typical mongoloid look; hence,it is also called mongoloid idiocy.It occurs due to the phenomenon of nondisjunction.Nondisjunction occurs when a pair of homologous chromosomes does not separate during meiosis but migrates to the same pole of the cell,resulting in an uneven number of chromosomes in the daughter cells ($45$ in one and $47$ in the other).This numerical abnormality results in trisomy $(2n + 1)$ and monosomy $(2n - 1)$.Nondisjunction is more common in sex chromosomes.

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