Genetics and Genomics: Transmission, Variation, and Genome-Scale Reasoning
Core concepts
Master Mendelian ratios and their deviations: incomplete dominance, codominance, epistasis (recessive, dominant, duplicate), and linkage with recombination frequency as a map proxy. Know sex-linked inheritance patterns and how to distinguish them from autosomal traits by pedigree. At the molecular level, understand mutation classes (missense, nonsense, frameshift, splice-site), DNA repair pathways (mismatch, base excision, nucleotide excision, double-strand break repair by HR or NHEJ), and the difference between germline and somatic variation. For genomics, be comfortable with sequencing depth, coverage, variant calling filters, Hardy-Weinberg equilibrium as a null model, and the meaning of linkage disequilibrium and GWAS effect sizes.
Traps
Pedigree items often hinge on whether an affected father can transmit an X-linked allele to a son (he cannot). Ratio questions may hide lethality or a testcross versus self-cross distinction. In population genetics, allele versus genotype frequency is routinely swapped. In genomics, students confuse depth (reads per base) with breadth (fraction of genome covered) and forget that low allele fraction can reflect somatic mosaicism or contamination rather than sequencing error.
Worked rule of thumb
For a dihybrid cross with independent assortment, start from 9:3:3:1 and then modify only the class affected by the stated interaction-dominant epistasis collapses to 12:3:1, recessive epistasis to 9:3:4. Writing the modified ratio before reading the answer choices prevents arithmetic drift and saves time under exam pressure.
Study strategy
Practice converting between allele and genotype frequencies both directions, and always state your null hypothesis before evaluating a chi-square or HWE result.