Continuity and Change
This domain examines how genetic information is maintained, expressed and changed, and how populations evolve. The exam tests DNA replication, protein synthesis, mutation, cell division, inheritance, gene expression, natural selection and speciation.
Core rules and concepts
- DNA replication is semi-conservative: each new double helix contains one parental and one new strand, built by complementary base pairing.
- Protein synthesis involves transcription and translation. Codons specify amino acids; mutations may be silent, missense or nonsense depending on their effect on the polypeptide.
- Mitosis produces genetically identical daughter cells for growth and repair. Meiosis halves the chromosome number and generates variation through crossing over and independent assortment.
- Inheritance involves alleles, genotype and phenotype. Segregation separates alleles during meiosis; independent assortment applies to loci on different chromosome pairs or sufficiently far apart on the same pair.
- Natural selection changes allele frequencies. Speciation requires reproductive isolation; evidence comes from fossils, homologous structures, molecular sequences and observed change.
Worked example
In a cross AaBb × AaBb, assume the loci assort independently. The probability of offspring aa is 1/4. The probability of B_ is 3/4 because BB and Bb both show the dominant phenotype. Therefore P(aaB_) = 1/4 × 3/4 = 3/16. The multiplication rule is valid only because the loci are independent; linked loci would require recombination information.
Traps that cost marks
Do not describe mutation as directed by need. Dominant alleles are not necessarily more common. Do not confuse meiosis with mitosis, and do not apply independent assortment to linked genes without justification.