Number of chromosomes in Down's syndrome are

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(B) Down's syndrome is a genetic disorder caused by the presence of an extra copy of chromosome number $21$,a condition known as trisomy of chromosome

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$47$

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(B) Down's syndrome is a genetic disorder caused by the presence of an extra copy of chromosome number $21$,a condition known as trisomy of chromosome $21$.In a normal human cell,there are $46$ chromosomes ($44$ autosomes + $2$ sex chromosomes).Due to the trisomy of the $21^{	ext{st}}$ chromosome,the total count becomes $46 + 1 = 47$ chromosomes.Thus,a patient with Down's syndrome has $47$ chromosomes,represented as $45A + XX$ or $45A + XY$.

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