A 4-year-old boy born to a 28-year-old mother presents with mild developmental delay (IQ 65), subtle upslanting palpebral fissures, and a single transverse palmar crease. Echocardiography shows a small ventricular septal defect. Peripheral blood karyotype counting 25 cells reveals the pattern shown in the diagram marked **A** — a mixture of normal 46,XY cells and cells with trisomy 47,XY,+21. Which of the following statements best characterizes the pathogenesis and recurrence risk of this condition?