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Question

Out of the three copies of chromosome $$21$$ in a child, two have come from the mother. Based on this when did the nondisjunction event most likely occurred.


A
Maternal meiosis II
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B
Paternal meiosis II
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C
Maternal meiosis I
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D
Paternal meiosis I
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Solution

The correct option is C Maternal meiosis I
Down's syndrome is mainly caused due to an extra copy of chromosome 21. The extra copy is formed due to the non-disjunction of chromosomes during meiosis.  In this case, as the two chromosomes of the child has the same allele as the mother so it can be concluded that the non-disjunction has taken place during maternal meiosis with duplication of the maternal allele.
So, the correct option is C.

Biology

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