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

Answer 1

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.