Genetics and Evolution
Domain 7 covers inheritance, the molecular basis of inheritance and evolution. Questions test Mendelian ratios, DNA replication, transcription and translation, Hardy–Weinberg equilibrium and natural selection.
Core rules
- Mendel’s law of segregation: alleles separate during gamete formation. Law of independent assortment applies to genes on different chromosomes.
- Monohybrid cross Tt × Tt gives genotype 1 TT : 2 Tt : 1 tt and phenotype 3 dominant : 1 recessive. A test cross with tt reveals the unknown genotype.
- DNA replication is semiconservative; DNA polymerase adds nucleotides 5′ to 3′. Transcription makes mRNA from DNA; translation uses ribosomes, tRNA and codons.
- Evolution requires heritable variation and differential reproduction. Hardy–Weinberg: p² + 2pq + q² = 1 when no mutation, migration, selection or genetic drift.
- Natural selection acts on phenotypes, changing allele frequencies over generations.
Worked example
In a population, the recessive allele frequency q = 0.2. Find the frequency of heterozygotes under Hardy–Weinberg. p = 1 − q = 0.8. Heterozygote frequency = 2pq = 2 × 0.8 × 0.2 = 0.32, or 32%. Reasoning: Hardy–Weinberg assumes random mating and no evolutionary forces; p² is homozygous dominant, q² homozygous recessive, and 2pq heterozygous.
Traps
- Confusing segregation with independent assortment.
- Saying DNA polymerase synthesises DNA 3′ to 5′; it reads template 3′ to 5′ but builds 5′ to 3′.
- Treating acquired traits as heritable in evolution.
- Using Hardy–Weinberg when selection or migration is strong.