Genetics: Inheritance Patterns and Probability
Genetics questions test transmission patterns and probability. Start with Mendelian basics: alleles, dominance, segregation, independent assortment. Then recognize the non-Mendelian patterns-incomplete dominance, codominance, multiple alleles, pleiotropy, epistasis, imprinting, and mitochondrial inheritance. Pedigree analysis is mostly pattern recognition: autosomal dominant appears in every generation and affects both sexes; autosomal recessive skips generations and clusters in consanguinity; X-linked recessive affects males far more and never passes father to son; X-linked dominant affects both sexes but often more females, with affected fathers passing to all daughters.
Hardy–Weinberg is a reliable calculation: p + q = 1 and p² + 2pq + q² = 1 for a large, randomly mating population without selection, mutation, or migration. Use q² for affected frequency, q for allele frequency, and 2pq for carriers.
Traps: confusing carrier frequency with disease frequency; forgetting that a person with an autosomal dominant disease may have an unaffected parent if penetrance is incomplete; and assuming linked genes assort independently. Also know that mutations can be silent, missense, nonsense, or frameshift, and that frameshifts usually cause the most damage.
Worked example: an autosomal recessive disease affects 1 in 10,000. q² = 0.0001, so q = 0.01 and p = 0.99. Carrier frequency 2pq ≈ 2 × 0.99 × 0.01 = 0.0198, about 1 in 50.
Rule of thumb: for recessive disease, take the square root of the affected frequency to get q before doing anything else.