Genetics: Mendelian Inheritance, Chromosomal Basis and Molecular Genetics
Questions here are ratio-based and pathway-based: you must derive or recognise a Mendelian ratio and explain it, and you must trace information from DNA to protein with the correct enzymes and signals.
Core rules
- Law of dominance; law of segregation (monohybrid 3:1, test cross 1:1); law of independent assortment (dihybrid 9:3:3:1, test cross 1:1:1:1).
- Incomplete dominance (Mirabilis, snapdragon) gives 1:2:1 both genotypically and phenotypically. Codominance: both alleles expressed, as in I^A I^B blood group. Multiple alleles: I^A, I^B, i.
- Pleiotropy - one gene with several effects. Polygenic inheritance - additive genes giving continuous variation, as in wheat kernel colour.
- Chromosomal basis: Sutton and Boveri; Morgan's linkage work showed that genes on the same chromosome do not assort freely and that recombination frequency rises with distance.
- Molecular: antiparallel double helix, A=T and G≡C, semiconservative replication (Meselson–Stahl), transcription, translation; the code is triplet, degenerate, non-overlapping and comma-less, with AUG as initiator and UAA, UAG, UGA as stop codons.
Worked example
Cross RrYy × rryy. The first parent makes four gametes RY, Ry, rY, ry in equal numbers; the second makes only ry. Offspring are RrYy, Rryy, rrYy and rryy in 1:1:1:1 - the diagnostic result of a dihybrid test cross, confirming independent assortment.
Traps that cost marks
Applying independent assortment to linked genes; claiming incomplete dominance breaks the law of segregation (the genotypic ratio is unchanged, only the phenotype shifts); writing the code as overlapping; confusing the 1:2:1 genotypic ratio with the 3:1 phenotypic one.