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.
Maternal meiosis II
Paternal meiosis II
Maternal meiosis I
Paternal meiosis I
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.