Genetics and Evolution
Genetics and Evolution tests inheritance patterns, how allele frequencies change, and how populations diverge. Questions often require probability calculations, interpretation of pedigrees or crosses, and distinguishing evolutionary mechanisms from outcomes.
Core rules
- Mendel and probability: segregation separates alleles at one locus; independent assortment applies to unlinked loci. Use the product rule for independent events and the sum rule for mutually exclusive outcomes.
- Linkage: genes on the same chromosome can recombine by crossing over. Recombination frequency is proportional to distance and cannot exceed 50%; 50% indicates independent assortment.
- Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1. The model assumes no mutation, no selection, no migration, random mating, and a large population. Deviations suggest evolution or nonrandom mating.
- Mechanisms: mutation creates variation; selection acts on phenotypes; genetic drift changes allele frequencies by chance, strongest in small populations; gene flow moves alleles between populations.
- Speciation: allopatric speciation involves geographic isolation; sympatric speciation can involve polyploidy, habitat shifts, or assortative mating.
Worked example
An autosomal recessive disorder affects 1 in 10,000 newborns in a population assumed to be in Hardy–Weinberg equilibrium. q² = 0.0001, so q = 0.01 and p = 0.99. The carrier frequency is 2pq = 2(0.99)(0.01) = 0.0198, about 1 in 50. The reasoning matters: q² gives the affected genotype frequency, but carriers are heterozygous and are far more common than affected individuals. If the population were small or mating were nonrandom, this estimate would be less reliable.
Traps
- Assuming a dominant allele is common; dominance describes expression, not frequency.
- Using q instead of q² for the affected frequency in a recessive disorder.
- Reporting recombination frequencies above 50% as if they measured distance.
- Claiming that individuals evolve; populations evolve as allele frequencies change.
- Forgetting that genetic drift is random with respect to fitness, unlike selection.