Genetics and Evolution: Inheritance, Molecular Basis and Evolutionary Principles
Domain 8: Genetics and Evolution - what is tested
The exam tests Mendelian inheritance and deviations, sex-linked inheritance, DNA structure and replication, transcription and translation, and evolutionary mechanisms and evidence.
Core rules
- Mendel's laws: dominance, segregation and independent assortment. A monohybrid cross gives 3:1 phenotype and 1:2:1 genotype; a dihybrid cross gives 9:3:3:1.
- Test cross identifies genotype by crossing with homozygous recessive. Back cross is with a parent; test cross is a type of back cross.
- ABO blood groups show multiple alleles and codominance. Sex-linked traits are carried on sex chromosomes; X-linked recessive traits are more common in males.
- DNA is a double helix with antiparallel strands; A pairs with T and G with C. Replication is semiconservative. Transcription makes RNA; translation uses mRNA codons and tRNA anticodons at ribosomes.
- Evolution: natural selection, genetic drift, mutation, gene flow and isolation. Evidence includes fossils, homologous organs, embryology and molecular similarity.
Worked example
In a monohybrid cross between pure tall (TT) and pure dwarf (tt) pea plants, all F1 are Tt and tall. Selfing F1 gives F2 genotypes 1 TT : 2 Tt : 1 tt, so phenotypes are 3 tall : 1 dwarf. Reasoning: the dwarf allele is recessive, so it appears only when homozygous. A test cross of an unknown tall plant with tt would give all tall if TT, but 1 tall : 1 dwarf if Tt; this is how genotype is inferred.
Traps
Saying linked genes assort independently; confusing genotype and phenotype ratios; calling DNA replication conservative; mixing transcription and translation locations in eukaryotes; using 'survival of the fittest' as a tautology; ignoring genetic drift in small populations.