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.

Column-$I$ (Disorder)	Column-$II$ (Ploidy)
$P$. Down's syndrome	$I$. Sex chromosomal trisomy
$Q$. Klinefelter's syndrome	$II$. Sex chromosomal monosomy
$R$. Turner's syndrome	$III$. Autosomal trisomy

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Column-$I$ (Disorder) Column-$II$ (Ploidy)

$P$. Down's syndrome $I$. Sex chromosomal trisomy

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