Preparatory Mindset

Mendelian patterns follow predictable rules. Master the pedigree: vertical transmission = AD, horizontal = AR, affected males through carrier females = XR. Always calculate recurrence risks — this is frequently tested.

Core Concepts

• Autosomal Dominant (AD) : Affects males and females equally, appears in every generation, male-to-male transmission possible, 50% risk to offspring of affected individual


• Autosomal Recessive (AR) : Affects males and females equally, usually one generation, 25% risk when both parents are carriers, increased with consanguinity


• X-linked Recessive (XR) : Affected males (no male-to-male transmission), carrier females, 50% of sons of carrier affected, 50% of daughters carriers


• X-linked Dominant (XD) : Affected females transmit to 50% of ALL children; affected males transmit to ALL daughters but NO sons


• Penetrance: Proportion of individuals with genotype who express phenotype (all-or-none). Complete vs incomplete.


• Expressivity: Degree or severity of phenotype — can vary widely (e.g., NF1: mild to severe)


• New mutation: AD disorder appearing with no family history → 50% transmission risk to offspring


• Gonadal mosaicism: Mutation present in some germ cells → recurrence risk higher than expected for new mutation


• Huntington disease: AD, complete penetrance, onset 30-50 years, CAG trinucleotide repeat


• CF, sickle cell, Tay-Sachs, PKU: Classic AR disorders with known carrier frequencies

High-Yield Points

• AD = vertical transmission, 50% risk, male-to-male possible


• AR = 25% risk, consanguinity increases risk


• XR = no male-to-male, carrier females, affected males


• Penetrance = binary (yes/no); Expressivity = severity (how much)

Topic Summary

Mendelian inheritance follows predictable patterns. Pedigree analysis reveals the mode. Recurrence risks depend on the pattern — 50% for AD, 25% for AR.