Genetics and Evolution
What the domain tests
Mendelian ratios, deviations such as incomplete dominance and codominance, linkage and recombination, sex determination, mutation, Hardy–Weinberg equilibrium, and the evidence for evolution including natural selection and speciation.
Core rules
- Monohybrid F2 = 3:1 phenotype, 1:2:1 genotype; dihybrid F2 = 9:3:3:1 if genes assort independently.
- Test cross uses the homozygous recessive to reveal the genotype of a dominant phenotype; back cross is to either parent.
- Linked genes on the same chromosome do not assort independently; recombination frequency rises with distance and never exceeds 50%.
- Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1 in a large, random-mating population with no selection, mutation, migration or drift.
- Natural selection acts on existing variation; it does not create variation on demand.
Worked example
An autosomal recessive disorder affects 1 in 10,000 people. Estimate the carrier 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. The reasoning depends on assuming Hardy–Weinberg conditions; if the population is inbred or selected, the estimate changes.
Traps
- Do not call 2pq the disease frequency; q² is the affected frequency.
- In incomplete dominance the heterozygote is intermediate; in codominance both alleles are expressed, as in AB blood group.
- Recombination frequency of 50% means genes behave as unlinked, not that they are on different chromosomes.
- Mutation is random with respect to need; selection is not.