Genetics & Embryology — High-Yield — USMLE Step 1 Notes
Free, high-yield revision notes for USMLE Step 1. Read here, or drill the same material as questions and flashcards in the app.
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Inheritance patterns
- Autosomal dominant: vertical transmission, both sexes (Marfan, NF, Huntington, familial hypercholesterolaemia)
- Autosomal recessive: skip generations, consanguinity (cystic fibrosis, most metabolic diseases)
- X-linked recessive: males affected, no male-to-male (Duchenne, haemophilia, G6PD, Lesch-Nyhan)
- Mitochondrial: maternal transmission to all children (Leber, MELAS)
Trinucleotide repeat and imprinting
- Anticipation: earlier/worse each generation (Huntington CAG, fragile X CGG, myotonic dystrophy CTG)
- Prader-Willi (paternal deletion) vs Angelman (maternal deletion) — the same 15q, opposite parent
Key trisomies
- 21 Down: flat facies, endocardial cushion defect, early Alzheimer, duodenal atresia
- 18 Edwards: clenched fists, rocker-bottom feet
- 13 Patau: cleft lip/palate, polydactyly, holoprosencephaly
Embryology essentials
- Neural tube closes by week 4; folate deficiency → neural tube defects (raised AFP)
- Pharyngeal (branchial) arches: 1 (Meckel, CN V), 2 (CN VII), 3 (CN IX), 4 & 6 (CN X)
- Aortic arch derivatives: 3rd → carotids, 4th → aortic arch/subclavian, 6th → pulmonary/ductus
- Teratogens: ACE inhibitors, warfarin, valproate, isotretinoin, alcohol (fetal alcohol syndrome)
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