Genetics and Evolution
This domain tests transmission genetics (Mendel, probability, non-Mendelian patterns), molecular genetics, and the mechanisms that change allele frequencies over time. Expect Punnett-square and pedigree reasoning plus population-genetic calculations.
Mendelian Rules
- Law of segregation: alleles separate in meiosis. Law of independent assortment applies to unlinked genes.
- Monohybrid cross of heterozygotes → 3:1 phenotype, 1:2:1 genotype. Dihybrid → 9:3:3:1.
- A testcross uses a homozygous recessive partner to reveal an unknown genotype; use the product rule for independent events.
Extensions
- Incomplete dominance gives an intermediate phenotype; codominance expresses both; multiple alleles (ABO) and sex-linked genes produce unequal male/female ratios.
- Linked genes violate independent assortment; recombination frequency rises with distance.
Population Genetics and Evolution
- Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1, assuming no selection, mutation, drift, migration, or nonrandom mating.
- Mechanisms of change: natural selection, mutation, genetic drift (bottleneck, founder effect), gene flow, nonrandom mating.
- Speciation requires reproductive isolation, often allopatric; fitness means reproductive success, not strength.
Worked Example
An autosomal recessive disorder affects 1 in 10,000 people. Affected frequency = 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.
Reasoning: Hardy–Weinberg equilibrium must be assumed. The affected frequency estimates q², not q - a classic error is to treat the given number as q directly. Carriers greatly outnumber affected individuals when q is small.
Traps
- Assuming a dominant allele is the common one.
- Confusing carrier frequency with disease frequency.
- Applying Hardy–Weinberg where selection or drift is clearly operating.
- Describing resistance as arising "because" of a drug rather than being selected by it.