Genetics and Evolution
Core concepts
Genetics begins with Mendel: alleles segregate, and different genes assort independently. Use Punnett squares and probability (product rule for independent events). Distinguish genotype from phenotype, dominant and recessive, incomplete dominance, codominance, multiple alleles, and sex-linked traits. Meiosis creates variation through crossing over and independent assortment; nondisjunction produces aneuploid gametes. Mutations are the ultimate source of new alleles. Population genetics uses Hardy–Weinberg equilibrium (p + q = 1; p² + 2pq + q² = 1) as a null model; deviations suggest selection, drift, gene flow, mutation, or nonrandom mating. Evolution is change in allele frequencies over time, driven by natural selection, genetic drift (small populations), gene flow, and mutation. Speciation can be allopatric or sympatric; evidence includes fossils, comparative anatomy, biogeography, and molecular data.
Traps
Dominant does not mean most common. Carriers show a recessive phenotype but carry the allele. Hardy–Weinberg describes a non-evolving population and requires random mating, no selection, no drift, no gene flow, and no mutation. Do not confuse genotype frequency with allele frequency or mitosis with meiosis.
Worked example
In Aa × Aa, the offspring genotype ratio is 1 AA : 2 Aa : 1 aa, so P(aa) = 1/4. If a recessive condition affects 4% of a population (q² = 0.04), then q = 0.2, p = 0.8, and carrier frequency 2pq = 0.32. Rule of thumb: for independent events, multiply probabilities.