Introductory Biology: Systems, Genetics, and Populations
What the section covers
Biology questions test causal reasoning across systems. Core areas: membrane structure and transport (simple versus facilitated diffusion, active transport, osmosis and tonicity), cell signaling and the cell cycle, the central dogma with mutation consequences, Mendelian and non-Mendelian genetics with probability, population genetics (Hardy–Weinberg), evolution (selection types, drift, speciation), and organ-system physiology - nervous, endocrine, cardiovascular, respiratory, renal, digestive, immune, and reproductive - plus basic development.
Traps
- Confusing osmolarity with tonicity: a solution can be iso-osmotic yet still drive water movement if its solutes are permeant.
- Misplacing eukaryotic events: transcription in the nucleus, splicing before export, translation in the cytoplasm or on the rough ER.
- Treating dominant as more common or stronger - dominance describes phenotypic expression, not allele frequency.
- Assuming Hardy–Weinberg holds when selection, drift, migration, mutation, or nonrandom mating is present.
- Swapping sympathetic (increased heart rate, bronchodilation, dilated pupils) with parasympathetic.
- Forgetting that most nephron reabsorption happens in the proximal tubule, while ADH fine-tunes water in the collecting duct.
Rule of thumb
For genetics probabilities, multiply across independent loci and use a Punnett square within a locus. In a heterozygote monohybrid cross, each offspring independently has a 1/4 chance of the recessive phenotype.
Worked example
A recessive disease affects 1 in 10,000 people, so q² = 0.0001, q = 0.01, and p = 0.99. Carrier frequency is 2pq ≈ 2(0.99)(0.01) ≈ 0.02, about 1 in 50 - carriers vastly outnumber affected individuals.