Genetics and Molecular Biology
Inheritance and the gene
The exam tests Mendel's laws, monohybrid and dihybrid ratios, deviations from Mendelian ratios, linkage, DNA structure, replication, transcription, translation and the genetic code.
Core rules
- Mendel's laws are dominance, segregation and independent assortment.
- Monohybrid F2 phenotypic ratio is 3:1; dihybrid F2 is 9:3:3:1 when genes are unlinked.
- Incomplete dominance gives 1:2:1; codominance expresses both alleles; multiple alleles occur at one locus, as in ABO blood groups.
- Linkage reduces recombination; crossing over produces recombinants.
- DNA is a double helix, antiparallel, with A-T (two hydrogen bonds) and G-C (three hydrogen bonds).
- Replication is semi-conservative; transcription makes RNA; translation makes protein.
- The genetic code is triplet and degenerate; AUG is the start codon; UAA, UAG and UGA are stop codons.
Worked example
Cross Tt × Tt. Reasoning: each parent forms T and t gametes in equal frequency; a Punnett square gives 1 TT : 2 Tt : 1 tt. TT and Tt are tall, tt is dwarf, so the phenotypic ratio is 3 tall : 1 dwarf. A test cross Tt × tt gives 1 tall : 1 dwarf.
Traps
9:3:3:1 requires independent assortment; linked genes deviate. Codominance is not incomplete dominance. Segregation concerns one pair of alleles; independent assortment concerns two or more pairs. DNA replication is semi-conservative, not conservative.