Biology: Levels of Organisation and Reasoning
What is assessed
Biology questions move across levels of organisation: molecules, cells, tissues, organs, organisms, populations, ecosystems. You should understand cell structure and division, genetics and inheritance, the principles of evolution, human physiology (respiration, circulation, digestion, nervous and hormonal control), and basic ecology including energy flow and nutrient cycles.
Traps that cost marks
- Confusing mitosis and meiosis: meiosis halves the chromosome number and produces genetically varied gametes; mitosis produces identical cells.
- Dominant versus frequent: a dominant allele is not necessarily the common one in a population.
- Direction of energy flow: energy flows through ecosystems and is lost as heat; matter cycles.
- Osmosis: water moves toward the higher solute concentration, not the higher water concentration.
- Correlation in ecology: an observed association may reflect a third factor.
Worked example
Cross two heterozygous parents, Aa × Aa, for a trait showing complete dominance. Gametes: each parent gives A or a with equal probability. Offspring: AA, Aa, Aa, aa, so the genotypic ratio is 1:2:1 and the phenotypic ratio is 3 dominant : 1 recessive. If the trait were sex-linked, the ratios would differ between sons and daughters, which is the usual trap in exam questions.
Rule of thumb
For any genetics question, write the parental genotypes and the gametes first, then draw the grid. Most errors come from skipping this step.