Genetics: Inheritance, DNA, Gene Expression and Genetic Engineering
Transmission, molecules, engineering
Genetics spans three layers: Mendelian transmission and probability, molecular expression from DNA to protein, and applied techniques. The exam often links them in one item, for example a mutation that changes a phenotype and is then detected by PCR.
Core rules
- Crosses: Aa × Aa gives 3:1 phenotype and 1:2:1 genotype; a test cross uses the recessive homozygote; a dihybrid cross of unlinked genes gives 9:3:3:1.
- Variations: incomplete dominance blends, codominance expresses both alleles, ABO has multiple alleles (I^A, I^B, i), X-linked recessive traits appear mostly in males.
- Replication: semiconservative; helicase unwinds; DNA polymerase adds only at the 3-prime end, so the leading strand is continuous and the lagging strand is made as Okazaki fragments joined by ligase.
- Expression: transcription uses RNA polymerase at a promoter, and eukaryotes splice out introns; translation starts at AUG, uses tRNA anticodons, and ends at a stop codon.
- Mutations: substitution (silent, missense, nonsense) versus insertion/deletion (frameshift).
- Engineering: restriction enzymes cut palindromic sites and leave sticky ends; ligase joins; a plasmid vector needs an origin, promoter and selection marker; PCR uses about 95 °C to denature, 55–60 °C to anneal primers and 72 °C for Taq polymerase extension; gel electrophoresis separates by size, with DNA running toward the anode; CRISPR-Cas9 targets DNA using a guide RNA.
Worked example
Two carrier parents (Aa × Aa): the chance of an affected child is 1/4 and of a carrier child 1/2. The chance that both of two children are affected is 1/4 × 1/4 = 1/16. Reasoning: separate pregnancies are independent events, so probabilities multiply.
Marks lost
Equating dominant with common; mixing up where transcription and translation occur; forgetting that primers define the amplified region; claiming electrophoresis separates by charge alone.