Unit 4: Heredity and continuity of life

Gene expression. Covers transcription (RNA polymerase, template strand, mRNA), the codon to amino acid code (universal, degenerate, non-overlapping), translation at the ribosome (initiation, elongation, termination) and the main differences between prokaryotic and eukaryotic expression.

DNA. Walks through the double-helix structure (sugar, phosphate, four bases, complementary pairing, antiparallel strands), the semi-conservative model demonstrated by Meselson and Stahl, and the roles of helicase, primase, DNA polymerase III, DNA polymerase I and ligase on the leading and lagging strands.

Mutations and variation. Covers point mutations (silent, missense, nonsense), frameshift indels, chromosomal mutations (deletion, duplication, inversion, translocation, non-disjunction) and the three sources of variation (independent assortment, crossing over, random fertilisation) plus mutation as the ultimate source.

Non-Mendelian inheritance. Walks through codominance (ABO blood groups, roan cattle), incomplete dominance (snapdragon flower colour), multiple alleles (ABO, coat colour), X-linked inheritance (haemophilia, colour blindness, Punnett squares with sex chromosomes), and polygenic inheritance (skin colour, height) with continuous variation.

Pedigree analysis. Explains pedigree symbols, generation and individual numbering, the four inheritance patterns and the signature clues for each (skipped generations, sex bias, affected fathers and daughters), and works through probability calculations using the product and sum rules for combined events.

Mendelian genetics. Defines genotype, phenotype, allele, homozygous and heterozygous, applies the laws of segregation and independent assortment to monohybrid and dihybrid Punnett squares (3:1 and 9:3:3:1 ratios), and explains how a test cross with a homozygous recessive parent reveals the genotype of an unknown dominant phenotype.

Natural selection. Covers the four preconditions (variation, heritability, differential survival and reproduction), defines fitness as reproductive success, distinguishes Darwin's theory from the neo-Darwinian synthesis (Mendelian genetics, mutation, population genetics) with examples in peppered moths, bacteria and cane toads.

Biotechnology. Covers PCR (denaturation, annealing, extension, Taq, primers), gel electrophoresis (charge, size, ladder), recombinant DNA (restriction enzymes, plasmids, ligase, transformation), transgenic organisms (Bt cotton, golden rice, recombinant insulin) and CRISPR-Cas9 (guide RNA, PAM, repair pathways).