Genetics and Evolution
Inheritance
Mendel's laws: segregation (alleles separate in meiosis) and independent assortment (genes on different chromosomes assort independently). Use Punnett squares and probability: multiply for independent events, add for mutually exclusive outcomes. Distinguish genotype from phenotype, and complete dominance from incomplete dominance (blended heterozygote) and codominance (both traits expressed). Trap: dominant does not mean more common in a population.
Molecular genetics
Central dogma: DNA → RNA → protein. Replication is semiconservative; transcription makes mRNA; translation at ribosomes uses codons and tRNA. Mutations: substitution (often silent due to redundancy), insertion/deletion (frameshift). Sex-linked traits sit on sex chromosomes; recessive X-linked disorders appear more often in males because they have one X.
Evolution
Evolution is change in allele frequencies across generations. Mechanisms: natural selection, mutation, gene flow, genetic drift (bottleneck, founder effects), and nonrandom mating. Natural selection acts on phenotypes and requires heritable variation. Hardy–Weinberg equilibrium (p² + 2pq + q² = 1) describes a non-evolving population; violations signal evolution. Speciation often requires reproductive isolation, with geographic isolation a common route. Fitness is reproductive success, not strength.
Worked example
Cross Aa × Aa. Offspring genotypes: 1 AA : 2 Aa : 1 aa; phenotypes with complete dominance: 3 dominant : 1 recessive. Probability of an aa offspring is 1/4; probability of two aa offspring in a row is 1/4 × 1/4 = 1/16. Rule of thumb: individuals do not evolve - populations do.