Core Ratios: Monohybrid F2 = 3:1 phenotypic (1:2:1 genotypic). Dihybrid F2 = 9:3:3:1. Incomplete/co-dominance F2 = 1:2:1. Test cross = 1:1. Dihybrid test cross = 1:1:1:1.

ABO Genetics: 3 alleles ($I^A$, $I^B$, i) → 6 genotypes, 4 phenotypes. AB = universal recipient (no antibodies). O = universal donor (no antigens). AB × O → only A and B children (no O or AB possible).

Key Distinctions: Incomplete dominance = blended phenotype (pink snapdragon). Co-dominance = both phenotypes (AB blood). Pleiotropy = 1 gene → many traits (HbS). Polygenic = many genes → 1 continuous trait (skin colour). Linked genes violate Independent Assortment.

Key Scientists: Mendel (laws, peas, 1860s) → Sutton + Boveri (chromosomal theory, 1902) → Morgan (linkage, Drosophila, 1910s). Sturtevant (first genetic map, 1913).

Modified Dihybrid Ratios (all sum to 16): 9:7 complementary | 9:3:4 recessive epistasis | 12:3:1 dominant epistasis | 15:1 duplicate | 13:3 inhibitory. Standard 9:3:3:1 only when independent assortment + complete dominance + no gene interaction.