Genetics and Evolution
Genetics and Evolution
This domain combines inheritance, molecular genetics, and evolutionary mechanisms. The exam asks you to trace information from DNA to phenotype, solve inheritance problems, and explain how populations change over time. Expect questions on replication, transcription and translation, Mendelian ratios, mutations, natural selection, and evidence for evolution.
Core rules and concepts
- DNA replication is semiconservative; transcription makes mRNA from DNA; translation uses ribosomes, tRNA, and codons to build polypeptides.
- Mendelian crosses follow segregation and independent assortment. A monohybrid Tt x Tt cross gives a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio when complete dominance applies.
- Nondisjunction produces aneuploid gametes; mutations can be silent, missense, nonsense, or frameshift.
- Natural selection acts on heritable variation; fitness is reproductive success in a given environment, not strength.
- Hardy-Weinberg equilibrium requires no mutation, no gene flow, no genetic drift, random mating, and no selection. Deviations suggest evolution is occurring.
- Evidence for evolution includes fossils, homologous structures, embryology, biogeography, and molecular sequence comparisons.
Worked example
In pea plants, tall (T) is dominant to short (t). A tall plant of unknown genotype is crossed with a short plant, and the offspring are 50% tall and 50% short. The short parent must be tt because short is recessive. If the tall parent were TT, all offspring would receive T and be tall. The 1:1 ratio means the tall parent produced T and t gametes in equal frequency, so its genotype is Tt. The reasoning uses the offspring phenotypes to infer the unknown parent's gametes, not just the parent's appearance.
Traps that cost marks
- Assuming a dominant allele is always more common in a population.
- Confusing genotype and phenotype ratios.
- Treating natural selection as goal-directed or as acting on individuals within a lifetime.
- Forgetting that Hardy-Weinberg describes a non-evolving population.