Genetics and Molecular Biology
Core concepts
Master DNA structure (double helix, antiparallel strands, complementary base pairing A–T, G–C), semi-conservative replication, transcription, and translation (codons, anticodons, ribosomes, start/stop codons). Know the genetic code is degenerate and non-overlapping. Be able to solve Mendelian crosses: monohybrid and dihybrid ratios, test crosses, codominance, incomplete dominance, sex-linked inheritance, and pedigree analysis. Understand mutation types (substitution, insertion, deletion, frameshift), the central dogma, and basic recombinant DNA tools (restriction enzymes, plasmids, PCR).
Traps in questions
- A frameshift arises from insertion/deletion of a number of bases not divisible by three - not from substitution.
- In dihybrid crosses, the 9:3:3:1 ratio requires independent assortment; linked genes distort it.
- Sex-linked recessive traits appear more often in males (one X); a carrier mother × unaffected father gives 50% affected sons.
- Do not confuse genotype ratio with phenotype ratio (1:2:1 vs. 3:1 for a monohybrid cross).
- Transcription makes mRNA from the template strand; the coding strand has the same sequence as mRNA (T→U).
Worked example / rule of thumb
Cross AaBb × AaBb (independent assortment): each gene gives 3:1 dominant:recessive, so phenotype ratio = 9:3:3:1. For a pedigree, test the simplest hypothesis first (autosomal recessive), then check whether sex-linked or dominant fits better - the correct mode must explain every affected individual. Rule of thumb: count the ratio, then map it to genotypes; if a ratio is unexpected, suspect linkage or lethality.