Monohybrid Cross Ratios (Complete Dominance)
| Cross | F1 Genotype | F2 Phenotypic | F2 Genotypic |
|---|---|---|---|
| TT × tt | Tt (all) | 3:1 | 1:2:1 |
| Tt × tt (test cross) | 1 Tt : 1 tt | 1:1 | 1:1 |
| TT × Tt | 1 TT : 1 Tt | All dominant | 1:1 |
Dihybrid Cross Ratios (Complete Dominance)
| Cross | F2 Phenotypic Ratio |
|---|---|
| TtRr × TtRr | 9:3:3:1 |
| TtRr × ttrr (test cross) | 1:1:1:1 |
Modified Dihybrid F2 Ratios (from AaBb × AaBb)
| Gene Interaction | Ratio | Example |
|---|---|---|
| Complementary genes | 9:7 | Flower colour in sweet peas |
| Supplementary / Recessive epistasis | 9:3:4 | Coat colour in mice/Labradors |
| Dominant epistasis | 12:3:1 | Fruit colour in squash |
| Duplicate genes | 15:1 | Kernel colour in wheat |
| Inhibitory genes | 13:3 | Feather colour in fowl |
Key Formulae
- Number of gamete types = 2^n (n = number of heterozygous gene pairs)
- Number of genotypes in F2 = 3^n
- Number of phenotypes in F2 = 2^n (complete dominance)
- Recombination frequency (%) = × 100
- 1 centimorgan (cM) = 1% recombination frequency = 1 map unit
- ABO genotypes = n = 3×4/2 = 6 (for n=3 alleles)
Probability Rules
- P(specific genotype in F2) = product of individual locus probabilities
- P(aabb from AaBb × AaBb) = 1/4 × 1/4 = 1/16
- P(A_B_ from AaBb × AaBb) = 3/4 × 3/4 = 9/16
- P(Aa offspring from Aa × Aa) = 1/2 (= 2/4)