Down Syndrome & the Autosomal Trisomies: 21, 18, 13

Three autosomal trisomies survive to birth, and the exam wants the split: 21 Down, 18 Edwards, 13 Patau. Nondisjunction in maternal meiosis I is the mechanism for the overwhelming majority, which is why maternal age drives the risk. This is part of the free Genetics series on Dr Shank: one topic, one decision tree, every branch worked with a real vignette. Dr Shank teaches USMLE Step 1 the way it should be taught: one topic, one decision tree, every single time. No fluff, no 45-minute lecture replays: just the exact branch-point logic the exam is testing, taught fast and taught for free. ▶ Every topic is built as one connected map, so each video snaps onto the one before it. All free. No paywall on the teaching. #USMLE #Step1 #MedSchool #USMLEStep1 #MedStudent Dr Shank teaches USMLE Step 1 the way it should be taught: one topic, one decision tree, every single time. No fluff, no 45-minute lecture replays: just the exact branch-point logic the exam is testing, taught fast and taught for free. ▶ Every topic is built as one connected map, so each video snaps onto the one before it. All free. No paywall on the teaching.