Genetics and Evolution
Genetics and Evolution
This domain tests inheritance, molecular genetics, and evolutionary mechanisms. Questions ask you to predict offspring ratios, interpret pedigrees, calculate allele frequencies, distinguish mutation from selection, and explain how reproductive isolation produces species.
Core rules and concepts
- Mendel's law of segregation: two alleles separate during gamete formation. Independent assortment applies to genes on different chromosomes; linked genes do not assort independently.
- Complete dominance masks the recessive allele in heterozygotes. In incomplete dominance, the heterozygote is intermediate; in codominance, both alleles are expressed.
- Probability: multiply independent events; add mutually exclusive events. A monohybrid cross of heterozygotes gives a 3:1 phenotype ratio and a 1:2:1 genotype ratio.
- Hardy–Weinberg equilibrium requires no mutation, no migration, random mating, no selection, and large population size. Allele frequencies: p + q = 1; genotype frequencies: p² + 2pq + q² = 1.
- Natural selection acts on phenotypes and changes allele frequencies; mutation creates variation; genetic drift is random, and gene flow moves alleles between populations. Speciation requires reproductive isolation.
Worked example
In a population at Hardy–Weinberg equilibrium, 4% of individuals show a recessive trait, so q² = 0.04. Then q = 0.2 and p = 0.8. Heterozygote frequency is 2pq = 2(0.8)(0.2) = 0.32, or 32%. The dominant phenotype frequency is p² + 2pq = 0.64 + 0.32 = 0.96. Reasoning: the recessive phenotype frequency gives q² directly only under equilibrium assumptions; take the square root before calculating carriers.
Traps
Confusing allele frequency with genotype frequency; assuming a dominant allele is more common; using Hardy–Weinberg when selection or nonrandom mating is present; treating acquired traits as heritable; confusing genetic drift with natural selection; forgetting that linked genes violate independent assortment.