A 24-year-old primigravida at 14 weeks gestation presents with severe vaginal bleeding and passage of grape-like tissue. Ultrasound shows a "snowstorm" appearance with no identifiable fetal parts. The gross specimen demonstrates the characteristic **B** (bunch-of-grapes appearance) with hydropic chorionic villi. Which of the following karyotypes and pathogenic mechanisms best explains this complete hydatidiform mole?