Grade 12 Biology: Integrated Syllabus - Genetics, Molecular Biology, Biotechnology and Ecology
This section covers the Grade 12 Biology paper. It tests a combined botany–zoology syllabus built around heredity, molecular biology, biotechnology, evolution, ecology and human health, and it rewards precise mechanism over memorised labels. Expect questions that ask you to explain a process step by step, interpret a cross or a population, and justify conclusions.
Core rules and concepts
- Genetics: Mendel's laws. A monohybrid F2 gives a 3:1 phenotype ratio; a dihybrid F2 gives 9:3:3:1 only when the two genes assort independently. A test cross against a homozygous recessive identifies an unknown dominant genotype.
- Molecular biology: DNA replication is semiconservative; transcription copies the template strand into mRNA; translation matches codons to amino acids at the ribosome.
- Biotechnology: restriction enzymes cut palindromic sequences to leave sticky ends; PCR cycles through denaturation, annealing and extension; in gel electrophoresis DNA migrates toward the anode because its phosphate backbone is negative.
- Ecology: roughly 10 percent of energy passes to the next trophic level, so energy pyramids are always upright, unlike pyramids of number.
Worked example
In a population of 1,000 at Hardy–Weinberg equilibrium, 160 individuals show a recessive trait. The recessive phenotype frequency is q² = 0.16, so q = 0.40 and p = 1 − q = 0.60. Carriers are heterozygotes: 2pq = 2(0.60)(0.40) = 0.48, i.e. 480 people. The reasoning is that the recessive phenotype reveals q² directly; once p and q are known, p² + 2pq + q² = 1 gives every genotype frequency.
Marks-losing traps
- "Dominant" does not mean common, and allele frequency is not genotype frequency. Answering "carrier frequency" with q instead of 2pq loses the mark.
- Keep ratios and fractions distinct: 9:3:3:1 is a phenotype ratio; 1:2:1 is the genotype ratio for one gene.
- Segregation explains single-gene behaviour; independent assortment is what produces 9:3:3:1.
- Hardy–Weinberg assumes a large population, random mating, and no mutation, migration or selection; a question may ask which assumption a scenario breaks.
- Name the template strand in transcription and state the direction of migration in electrophoresis.