Heredity and Continuity of Life
Core concepts
Candidates must understand DNA structure and replication (semi-conservative), the central dogma (DNA → RNA → protein), transcription and translation, and how genes determine traits. Know the difference between genotype and phenotype, dominant and recessive alleles, and the patterns of inheritance: monohybrid and dihybrid crosses, codominance, incomplete dominance, sex-linked traits and multiple alleles (e.g. ABO blood groups).
Understand meiosis as a source of genetic variation through independent assortment and crossing over, and the role of mutation in generating new alleles. Study pedigree analysis and how to infer genotypes from phenotypes. Know the causes of variation - genetic and environmental - and how natural selection acts on heritable variation to drive evolution. Understand population genetics concepts such as allele frequency and the conditions that change it.
Traps
- A Punnett square gives probabilities, not guarantees - ratios describe expected outcomes over many offspring.
- Codominance shows both alleles (e.g. AB blood type); incomplete dominance shows a blended phenotype.
- Sex-linked (X-linked) recessive traits appear more often in males, who have one X.
- Mutation is the ultimate source of new alleles; crossing over and independent assortment rearrange existing alleles.
Worked example
Cross two heterozygotes Aa × Aa: offspring ratio is 1 AA : 2 Aa : 1 aa, i.e. 3 dominant : 1 recessive phenotype. Rule of thumb: multiply the probabilities of independent events for dihybrid crosses (e.g. 3/4 × 3/4 = 9/16).