Genetics, populations, evolution and ecosystems
Monohybrid and dihybrid inheritance
Monohybrid: one gene. Two heterozygotes (Aa × Aa) give 3 : 1 dominant : recessive. Codominance: both alleles expressed (e.g. Iᴬ Iᴮ blood group AB). Multiple alleles: more than two alleles in the population (Iᴬ, Iᴮ, Iᴼ). Sex linkage: gene on the X chromosome, so males (XY) show recessive traits more often (e.g. haemophilia). Dihybrid: two genes on different chromosomes; AaBb × AaBb gives 9 : 3 : 3 : 1; a test cross with aabb gives 1 : 1 : 1 : 1. Autosomal linkage: genes on the same chromosome are inherited together, giving more parental types and fewer recombinants than expected. Epistasis: one gene affects the expression of another, modifying ratios (e.g. 9 : 3 : 4). The chi-squared test checks whether differences from expected ratios are due to chance.
≈ 55 minNatural selection and population genetics
Variation may be discontinuous (distinct categories, few genes, e.g. blood group) or continuous (range, many genes plus environment, e.g. height). Natural selection: individuals vary; a selection pressure gives some a survival and reproductive advantage; they pass on their alleles, so allele frequencies change over generations. Stabilising selection favours the average; directional favours one extreme; disruptive favours both extremes. Genetic drift changes allele frequencies by chance, strongest in small populations (founder effect, bottleneck). Hardy–Weinberg: in a large, randomly mating population with no selection, mutation or migration, and , letting you estimate allele and carrier frequencies. Artificial selection is selective breeding by humans.
≈ 50 minSpeciation and evolution
A species (biological concept) is a group of organisms that can interbreed to produce fertile offspring. Speciation requires reproductive isolation, which stops gene flow. Allopatric speciation: a geographical barrier (river, mountain, sea) separates populations; different mutations, selection pressures and genetic drift make their allele frequencies diverge until they can't interbreed. Sympatric speciation happens in the same area through ecological, behavioural or temporal isolation, or polyploidy in plants. Isolating mechanisms may be prezygotic (different breeding seasons, courtship, incompatible genitalia) or postzygotic (hybrid inviability or sterility, e.g. mules). Molecular evidence (DNA base sequences, mitochondrial DNA, amino acid sequences of proteins such as cytochrome c) shows relationships: fewer differences mean more recent common ancestry.
≈ 45 minConservation and human impact
Biodiversity is worth conserving for ethical, economic (medicines, food, tourism), ecological (stable ecosystems) and aesthetic reasons. Threats: habitat loss, climate change, pollution, invasive species, overexploitation. In situ conservation protects species in their habitat (national parks, marine reserves, controlling invasive species, managing succession by grazing or coppicing). Ex situ: zoos (captive breeding, genetic diversity using studbooks), botanic gardens, seed banks (dried, frozen seeds), frozen zoos (sperm, eggs, embryos), assisted reproduction (IVF, embryo transfer). CITES controls international trade in endangered species; the Convention on Biological Diversity promotes sustainable use. Reintroduction needs suitable habitat and genetic variety.
≈ 40 min