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

Solution

## The correct option is C Maternal meiosis IDown'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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