Genetics and Evolution
Domain 2 - Genetics and Evolution spans inheritance patterns, molecular genetics, and the mechanisms that change allele frequencies over time. Expect questions that require you to predict ratios, interpret pedigrees, or decide whether a population is evolving.
Inheritance
- Law of segregation: homologous alleles separate in meiosis I. Independent assortment of unlinked genes gives the 9:3:3:1 dihybrid ratio.
- A testcross with a homozygous recessive individual reveals an unknown dominant genotype.
- Non-Mendelian patterns: incomplete dominance (blended heterozygote), codominance (both alleles expressed, as in AB blood), multiple alleles, epistasis, polygenic traits, and X-linked recessive inheritance (more affected males; fathers pass the allele only to daughters, mothers to sons).
- Linked genes depart from independent assortment; for short distances, 1% recombination equals 1 map unit (centimorgan).
Molecular genetics
- Replication is semiconservative; DNA polymerase adds nucleotides 5'→3', making the leading strand continuously and the lagging strand as Okazaki fragments.
- The lac operon is inducible: lactose binds the repressor and permits transcription. The trp operon is repressible.
- A frameshift from an insertion or deletion alters every downstream codon; a missense change swaps one amino acid; a nonsense change creates a premature stop.
Evolution
- Hardy–Weinberg (p² + 2pq + q² = 1) describes a non-evolving population: large size, random mating, no mutation, no migration, no selection.
- Mechanisms of change: natural selection, genetic drift (bottleneck, founder effect), gene flow, and mutation, producing directional, stabilizing, or disruptive selection.
- Reproductive isolating mechanisms lead to speciation; homology indicates common ancestry, analogy indicates convergence.
Worked example
If 16% of a population expresses a recessive trait, q² = 0.16, so q = 0.4 and p = 0.6. Heterozygotes are 2pq = 2(0.6)(0.4) = 0.48, meaning 48% are carriers. The reasoning depends on assuming Hardy–Weinberg equilibrium, so state that assumption before computing.
Traps that cost marks
- Assuming the dominant allele is the common one.
- Applying p² and q² to X-linked traits without allowing for male hemizygosity.
- Treating recombination frequency as linear over long distances, where double crossovers cause underestimation.