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

Options

A. Maternal meiosis II
B. Paternal meisosis I
C. Maternal meiosis I
D. Paternal meiosis I

Answer

To determine when the nondisjunction event most likely occurred in the case of a child with three copies of chromosome 21, where two copies are from the mother, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Nondisjunction**: Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division, which can lead to an abnormal number of chromosomes in daughter cells.

2. **Identifying the Chromosome Involved**: In this scenario, we are dealing with chromosome 21, which is associated with Down syndrome when there is an extra copy (trisomy 21).

3. **Analyzing the Source of Chromosomes**: The question states that out of the three copies of chromosome 21, two have come from the mother and one from the father. This indicates that the extra chromosome is maternal in origin.

4. **Determining the Timing of Nondisjunction**: Since two of the three copies of chromosome 21 are from the mother, we can conclude that the nondisjunction event must have occurred during the maternal meiosis. This is because the extra chromosome (the third copy) is not from the father.

5. **Conclusion**: Therefore, the nondisjunction event most likely occurred during maternal meiosis, leading to the child having two copies of chromosome 21 from the mother and one from the father.

### Final Answer:
The nondisjunction event most likely occurred during **maternal meiosis**.

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