Three copies of chromosome-21 in a child with | vedclass

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(B) The correct answer is $B$. Down's syndrome is an autosomal aneuploidy caused by the presence of an extra copy of chromosome-$21$. In this case,the

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Maternal meiosis-$I$

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(B) The correct answer is $B$. Down's syndrome is an autosomal aneuploidy caused by the presence of an extra copy of chromosome-$21$. In this case,the child has $3$ copies of chromosome-$21$,where $2$ copies share the same alleles as one of the mother's chromosomes. If non-disjunction occurs during meiosis-$I$,the homologous chromosomes fail to separate,resulting in a gamete containing both maternal homologs. Since these $2$ chromosomes are identical to the mother's alleles (due to the presence of homologous chromosomes that did not segregate),it indicates that the non-disjunction event occurred during maternal meiosis-$I$.

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