Biology: Cells, Genetics, Ecology and Evolution
Biology on this paper links molecular and cellular processes to inheritance, populations and evolutionary change. Questions often ask you to interpret data or a genetic cross and to explain the mechanism, not merely name a term.
Core rules
- Central dogma: DNA is transcribed to mRNA, which is translated at ribosomes; enzymes lower activation energy without changing the equilibrium.
- Mendelian crosses: use a Punnett square for one or two genes; a test cross reveals an unknown genotype. Linked genes deviate from independent assortment.
- Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1, assuming no selection, mutation, migration, drift or non-random mating.
- Ecology: energy decreases along trophic levels, roughly 10% transferring to the next; population growth is exponential when resources are unlimited and logistic when density dependence operates.
- Evolution: natural selection changes allele frequencies; genetic drift is random, and speciation requires reproductive isolation.
Worked example
A recessive disorder affects 1 in 10 000 people in a population at Hardy–Weinberg equilibrium. Then q² = 1/10 000, so q = 0.01 and p = 0.99. The carrier frequency is 2pq = 2(0.99)(0.01) = 0.0198, about 2%. The key step is recognising that the affected frequency is q², not q, and that carriers are heterozygous.
Traps
Reporting q² as the carrier frequency, applying Hardy–Weinberg where selection is stated, confusing mitosis with meiosis, and calling a dominant allele 'more common' are common errors. In ecology, do not reverse the direction of energy flow or ignore decomposers.