Genetics and Molecular Biology
This domain tests Mendelian inheritance, deviations from simple ratios, sex determination and the molecular basis of gene expression. Expect pedigree, cross and molecular-biology questions.
Core rules
- Mendel laws: dominance, segregation and independent assortment. A test cross (Aa x aa) reveals an unknown genotype.
- Deviations: incomplete dominance gives intermediate heterozygotes; codominance expresses both alleles; ABO blood group is multiple allelic; epistasis involves one gene masking another; polygenic traits show continuous variation.
- Linkage: genes on the same chromosome tend to be inherited together; crossing over produces recombinants. Recombination frequency measures map distance.
- Sex determination: XX/XY, ZW/ZZ and haplodiploidy. Sex-linked genes are on sex chromosomes; a Barr body is an inactivated X.
- Molecular central dogma: DNA replication is semiconservative; transcription makes RNA (U replaces T); translation at ribosomes uses mRNA codons and tRNA anticodons. The lac operon is inducible; the trp operon is repressible.
Worked example
In a dihybrid cross AaBb x AaBb with independent assortment, what fraction is A_bb? Reasoning: probability of A_ is 3/4 and bb is 1/4, so the product is 3/16. The full phenotypic ratio is 9:3:3:1 only if the genes assort independently and show complete dominance.
Traps
9:3:3:1 is not universal. Linkage reduces recombinant frequency. Transcription uses uracil, not thymine. Translation starts at AUG. A Barr body is not a Y chromosome.