A 3-year-old boy with Down syndrome is referred for genetic counseling. Karyotyping reveals the pattern shown as **B** in the diagram — a mixture of cells with normal chromosome complement (46,XY) and cells with trisomy 21 (47,XY,+21). The child has mild hypotonia, characteristic facial features, and an IQ in the low-normal range. His parents' karyotypes are normal. Which of the following best explains the mechanism of the condition marked **B** and the recurrence risk in future pregnancies?
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